Prof. Mike Merrick

John Innes Centre, Norwich Research Park, Colney, Norwich, Norfolk, NR4 7UH, UK

Tel: +44 (0)1603 450749
Fax: +44 (0)1603 450778

Research Interests

Research in my laboratory is focussed on bacterial nitrogen metabolism and the ways in which bacteria control all aspects of that metabolism in response to the availability of fixed nitrogen. Bacteria can use a wide range of organic and inorganic sources of nitrogen and they must therefore coordinate both the expression of genes and the activities of proteins required for nitrogen metabolism with the availability of nitrogen sources in their environment and with their intracellular nitrogen status (for reviews see: Merrick and Edwards, 1995; Arcondeguy et al, 2001).

Current Projects

Ammonium transport

The transport of ammonium across cell membranes is a process of fundamental importance in almost all living organisms. However the means by which ammonium enters cells was the subject of debate for many decades until genes encoding high-affinity ammonium transporters (Amt) were isolated in 1994. The Amt family of proteins is unique and ubiquitous, being found in eubacteria, archaebacteria, fungi, plants, nematode worms and insects. In 1997 it was recognised that members of the Amt family are also present in higher animals including humans where their homologues are the Rhesus proteins.

In our laboratory we have developed the AmtB protein of Escherichia coli as a model which offers an excellent system to investigate questions of structure, function and signal transduction relating to Amt proteins (Merrick et al., 2006).

Organophosphate degradation

We are also collaborating in studies on the biology of organophosphate degradation by soil bacteria, which may have significant applications in bioremediation.

Curriculum Vitae

B.Sc: Genetics, University of Birmingham, UK (1970)
Ph.D. Genetics, University of Birmingham, UK (1973)
Postdoc: John Innes Centre, Norwich UK (1973-76)
Research Scientist: Nitrogen Fixation Laboratory, University of Sussex, UK (1976 - 1995)
Project Leader: Dept. of Molecular Microbiology, John Innes Centre, Norwich, UK (1995 - present)
Associate Head of Department (2001- )

Ammonium Transport

The transport of ammonium across cell membranes is a process of fundamental importance in almost all living organisms. However the means by which ammonium enters cells was the subject of debate for many decades until genes encoding high-affinity ammonium transporters (Amt) were isolated in 1994 from both Saccharomyces cerevisae and Arabidopsis thaliana. The Amt family of proteins is unique and ubiquitous, being found in eubacteria, archaebacteria, fungi, plants, nematode worms and insects. In 1997 it was recognised that members of the Amt family are also present in higher animals including humans where their homologues are the Rhesus proteins.

The Amt and Rh protein family
The Amt/Rh protein family.

E. coli AmtB structure and function

The membrane topology of the E. coli Amt protein (AmtB) provides a model for the whole Amt family (Thomas, Mullins and Merrick, 2000). Bioinformatic data indicates that the majority of Amt proteins have eleven trans-membrane helices (TMH) with the N-terminus outside the membrane and the C-terminus inside. A subset of proteins, including E.coli AmtB, has an apparent additional twelfth TMH which has subsequently been shown to be a signal peptide (Thornton et al., 2006). The presence of this signal peptide appears to characterise Amt proteins in Gram –ve bacteria but its precise role in maturation of AmtB remains to be determined.

Purification and characterisation of E. coli AmtB showed it to be a stable trimer and this is almost certainly the case for all members of the Amt family (Blakey et al., 2002). The availability of purified AmtB facilitated the production of two-dimensional crystals that confirmed the trimeric nature of the protein (Conroy et al., 2004). Three dimensional crystals subsequently led to the solution of the X-ray structure of E. coli AmtB (Zheng et al., 2004; Khademi et al., 2004). The structure of AmtB strongly suggests that the protein functions as a channel (rather than a transporter), that mediates the flux of ammonia, NH₃, through the conduction pore. Current models suggest that Amt proteins bind ammonium, NH₄+, in the outer vestibule and subsequently deprotonate it. Whether the proton released also crosses the membrane or not remains to be determined (Javelle et al., 2006; Javelle et al., 2007, Javelle et al., 2008).

The E. Coli AmtB protein (Zheng et al., 2004)
The E. Coli AmtB protein.

Amt proteins and signal transduction

Amt proteins have also been implicated in cellular responses to ammonium availability (i.e. ammonium sensing) in a variety of organisms, including bacteria such as Azospirillum brasilense, fungi such as Saccharomyces cerevisiae and Ustilago maydis, and the slime mould Dictyostelium discoideum.

In bacteria the amtB gene is almost invariably transcriptionally linked to a second gene, glnK, that encodes a signal transduction protein (GlnK) belonging to the PII family (Arcondeguyet al.,2000). The conserved linkage of these two genes is strongly suggestive of a functional interaction between their products (Thomas, Coutts and Merrick, 2000) and indeed we have shown that in N-sufficient GlnK is sequestered to the membrane in an AmtB-dependent fashion (Coutts et al., 2002; Javelle et al., 2004). The AmtB-GlnK complex can be purified (Durand and Merrick, 2006) and its X-ray structure has been solved (Conroy et al., 2007). This structure demonstrates that binding of GlnK to AmtB physically blocks the ammonia conduction channel thereby controlling ammonia influx into the cell. This interaction is likely to be conserved throughout prokaryotes.

The E. coli AmtB – GlnK complex (Conroy et al., 2007)
X-ray structure of the E. coli AmtB/GlnK complex

In some cases the GlnK-AmtB complex may be physically integrated into other nitrogen regulatory mechanisms within the cell. For example some nitrogen-fixing bacteria can reversibly inactivate the nitrogenase enzyme in response to extracellular ammonium. This process involves ADP-ribosylation of the NifH subunit of nitrogenase by the concerted efforts of inactivating (DraT) and activating (DraG) enzymes. In A. brasilense the activities of these enzymes is controlled by their interactions with two PII proteins, GlnB and GlnK (Huergo et al., 2006a; Huergo et al., 2006b). The cellular location of the DraG enzyme is altered in response to ammonium availability as a consequence of its sequestration to the membrane in a ternary complex with GlnK and AmtB in N-sufficient conditions (Huergo et al., 2007). The formation of such ternary complexes between AmtB, GlnK and a GlnK target protein may constitute a new mechanism of nitrogen regulation that could occur in other prokaryotes.

Lab Publications on ammonium transport

Bao-zhen, L., Merrick, M., Su-mei, L., Hong-ying, L., Shu-wen, Z., Wei-ming, S. and Yan-hua, S. (2009) Molecular Basis and Regulation of Ammonium Transporter in Rice. Rice Science 16(4): 314-322 (pdf file)

Javelle, A., Lupo, D., Ripoche, P., Fulford, T., Merrick, M. and Winkler, F.K. (2008) Substrate binding, deprotonation and selectivity at the periplasmic entrance of the E. coli ammonia channel AmtB. Proc. Natl. Acad. Sci. USA. 105: 5040-5045 (pdf file).

Javelle, A., Lupo, D., Li, X-D., Merrick, M., Chami, M., Ripoche, P. and Winkler, F.K. (2007) Structural and mechanistic aspects of Amt/Rh proteins. J. Struct. Biol. 105: 5040-5045 (pdf file).

Conroy, M.J., Durand, A., Lupo, D., Li-X-D., Bullough, P.A., Winkler, F.K. and Merrick, M. (2007) The crystal structure of the Escherichia coli AmtB-GlnK complex reveals how GlnK regulates the ammonia channel. Proc. Natl. Acad. Sci. USA. 104: 1213-1218 (pdf file).

Severi, E., Javelle, A. and Merrick, M. (2007) The conserved carboxy-terminal region of the ammonia channel AmtB plays a critical role in channel function. Mol. Memb. Biol. 24:161-171 (pdf file).

Javelle, A., Lupo, D., Zheng, L., Li, X-D., Winkler, F.K. and Merrick, M. (2006) An unusual twin-His arrangement in the pore of ammonia channels is essential for substrate conductance. J. Biol. Chem. 281: 39492-39498 (pdf file)

Huergo, L.F., Chubatsu, L.S., Souza, E.M., Pedrosa, F.O., Steffens, M.B.R. and Merrick, M. (2006) Interactions between PII proteins and the nitrogenase regulatory enzymes DraT and DraG in Azospirillum brasilense. FEBS Letts. 580: 5232-5236 (pdf file)

Durand, A. and Merrick, M. (2006) In vitro analysis of the Escherichia coli AmtB-GlnK complex reveals a stoichiometric interaction and sensitivity to ATP and 2-oxoglutarate J. Biol. Chem 281: 29558-29567. (pdf file)

Merrick, M., Javelle, A., Durand, A., Severi, E., Thornton, J., Avent, N.D., Conroy, M.J. and Bullough, P.A. (2006) The Escherichia coliAmtB protein as a model system for understanding ammonium transport by Amt and Rh proteins. Transfusion clinique et biologique 13: 97-102.(pdf file)

Thornton, J., Blakey, D., Scanlon, E. and Merrick, M. (2006) The ammonia channel protein AmtB from Escherichia coli is a polytopic membrane protein with a cleavable signal peptide. FEMS Microbiol. Letts. 258: 114-120 (request pdf file)

Huergo, L.F., Souza, E.M., Araujo, M.S., Pedrosa, F.O., Chubatsu, L.S., Steffens, M.B.R., and Merrick, M. (2006) ADP-ribosylation of dinitrogenase reductase in Azospirillum brasilense is regulated by AmtB-dependent membrane sequestration of DraG. Mol. Microbiol. 59: 326-337 (pdf file).

Conroy, M.J., Bullough, P.A., Merrick, M. and Avent, N.D. (2005) Modeling the human Rhesus proteins: implications for structure and function. Brit. J. Haematology 131: 534-551 (pdf file)

Javelle, A., Thomas, G., Marini, A-M., Krämer, R., and Merrick, M. (2005) In vivo functional characterisation of the E. coli ammonium channel AmtB: evidence for metabolic coupling of AmtB to glutamine synthetase. Biochem. J. 390: 215-222. (pdf file)

Javelle, A. and Merrick, M. (2005) Complex formation between AmtB and GlnK: an ancestral role in prokaryotic nitrogen control. Biochem. Soc. Trans. 33: 174-176. (pdf file)

Conroy, M.J., Jamieson, S.J., Blakey, D., Kaufmann, T., Engle, A., Fotiadis, D., Merrick, M. and Bullough, P.A. (2004) Electron and atomic force microscopy of the trimeric ammonium transporter AmtB. EMBO reports 5: 1153-1158. (pdf file)

von Wiren N. and Merrick, M.(2004) Regulation and function of ammonium carriers in bacteria, fungi and plants. Topics in Current Genetics 9: 95-120. (pdf file)

Javelle A., Severi E., Thornton J., and Merrick M. (2004) Ammonium sensing in E.coli : The role of the ammonium transporter AmtB and AmtB-GlnK complex formation J. Biol. Chem. 279: 8530-8538. (pdf file)

Blakey, D., Leech, A., Thomas, G.H., Coutts, G., Findlay, K. and Merrick, M.(2002) Purification of the Escherichia coli ammonium transporter AmtB reveals a trimeric stoichiometry. Biochem. J. 364: 527-535. (pdf file).

Coutts, G., Thomas, G., Blakey, D., and Merrick, M. (2002) Membrane sequestration of the signal transduction protein GlnK by the ammonium transporter AmtB. The EMBO Journal 21: 536-545 (pdf file).

Thomas, G.H., Mullins, J. G. L. and Merrick, M. (2000) Membrane topology of the Mep/Amt family of ammonium transporters. Molecular Microbiology 37: 331-344. (pdf file)

Thomas, G., Coutts, G. and Merrick, M. (2000) The glnKamtB operon: a conserved gene pair in prokaryotes. Trends in Genetics 16: 11-14. (pdf file)

Taté R., Cermola M., Riccio A., Iaccarino M., Merrick M., Favre R. and Patriarca E.J. (1999) The ectopic expression of the Rhizobium etli amtB gene affects the symbiosome differentiation process and nodule development. Molecular Plant-Microbe Interactions 12: 515-525.

Taté R., Riccio A., Merrick M. and Patriarca E.J. (1998) The Rhizobium etli amtB gene coding for an NH4+ transporter is down-regulated early during bacteroid differentiation. Molecular Plant-Microbe Interactions 11:188-198.

Rhesus Proteins

Rhesus proteins

Rhesus (Rh) proteins show significant similarity to Amt proteins and have also been demonstrated to also be capable of ammonium transport. The human Rh proteins (RhAG, RhCE, RhD) are found in the membranes of red blood cells and constitute a major class of blood group antigens. Their precise physiological role in the erythrocyte has yet to be established and indeed some investigators have argued that their function in the erythrocyte is to facilitate movement of CO₂, rather than ammonia, across the cell membrane. In humans two non-erythroid Rh proteins (RhBG, RhCG) are also present. These are expressed in the liver and kidney where they are proposed to play important roles in ammonium transport. In fish Rh proteins are found in not only in red blood cells, kidney and spleen but also in gills where they are thought to mediate ammonia excretion. Rh-like proteins are also found in nematodes, slime moulds and marine sponges and it seems likely that they could have a common role, namely the transport of ammonium, in all these organisms.

The human Rh erythrocyte complex has been proposed to be a tetramer but modelling of Rh proteins using the structure of E. coli AmtB as a starting point, led us to conclude that Rh proteins were also likely to be trimeric (Conroy et al., 2005). Recently genes encoding Rh proteins have been identified in some bacterial genomes. We have studied one of these, from an ammonia-oxidising bacterium Nitrosomonas europaea, and have solved the X-ray structure of the protein. NeRh50 is indeed a homotrimer which is structurally very similar to AmtB and when expressed in yeast it is able to facilitate ammonium uptake (Cherif-Zahar et al., 2007; Lupo et al., 2007).

The structure of the Nitrosomonas europaea Rh50 protein.

Lab Publications on Rhesus proteins

Lupo, D., Li, X-D., Durand, A., Tomizaki, T., Cherif-Zahar, B., Matassi, G., Merrick, M. and Winkler, F. (2007) The 1.3 Å resolution structure of Nitrosomonas europaea Rh50 and mechanistic implications for NH3 transport by Rhesus family proteins. Proc. Natl. Acad. Sci. USA. 104: 19303-19308. (pdf file)

Cherif-Zahar, B., Durand, A., Schmidt, I., Matic, I., Merrick, M. and Matassi, G. (2007) Evolution and Functional Characterisation of the RH50 Gene from the Ammonia-Oxidizing Bacterium Nitrosomonas europaea. J. Bacteriol. 189:, 9090-9100. (pdf file)

Javelle, A., Lupo, D., Li, X-D., Merrick, M., Chami, M., Ripoche, P. and Winkler, F.K. (2007) Structural and mechanistic aspects of Amt/Rh proteins. J. Struct. Biol.158: 472-481 (pdf file)

Conroy, M.J., Bullough, P.A., Merrick, M. and Avent, N.D. (2005) Modeling the human Rhesus proteins: implications for structure and function. Brit. J. Haematology 131: 534-551 (pdf file)

Organophosphate Degradation

Bacterial degradation of organophosphate pesticides

Organophosphorus (op) pesticides are used worldwide to control major insect pests. These pesticides inhibit acetlylcholinesterase (AchE) in an irreversible manner and cause insect death. As AchE is also present in all vertebrates, the potential for damage by this class of insecticides to non-target organisms is extremely high and the use of certain op-pesticides is now banned in most developed countries. However in many developing counties available alternative technologies for the effective replacement of op pesticides are limited and consequently they remain the major insecticides in agricultural pest management. This practice poses significant health hazards and op pesticide poisoning can occur in rural communities. Furthermore op pesticides do considerable damage to the ecosystem and are consequently a major threat to the practice of safe and sustainable agriculture.

For a number of years we have been collaborating with Professor D. Siddavatam (formerly at the Dept. of Biochemistry, SK University, Anantapur and now at Dept of Animal Sciences, School of Life Sciences, University of Hyderabad, India) in the study of a bacterium, Flavobacterium balustinum, isolated from agricultural soils in southern India and capable of utilising methylparathion as sole carbon source. The enzyme responsible for degradation is parathion hydrolase, the structural gene for which (opd) has been shown to be carried on a plasmid in this organism and to be closely related to other opd genes previously isolated in the USA and the Phillipines (Siddavattam et al., 2003). The opd gene is closely linked to a gene that we have designated mfhA and which encodes a novel meta fission product hydrolase (Khajamohiddin et al., 2006).

Publications relating to organophosphate degradation

Gorla, P., Pandey, J.P., Parthasarathy, S., Merrick, M. and Siddavattam, D. (2009) Organophosphate hydrolase in Brevundimonas diminuta is targeted to the periplasmic face of the inner membrane by the twin arginine translocation (Tat) pathway. J. Bacteriol. 191: 6292-6299.

Khajamohiddin, S., Repalle, E.J., Pinjari, A.B., Siddavattam, D. and Merrick, M. (2008) Biodegradation of aromatic compounds: An overview of meta-fission product hydrolases. Critical Reviews in Microbiology 34: 13-31.

Khajamohiddin, S., Pakala, S.B., Chakka, D.P., Merrick, M., Bhaduri, A., Sowdhamini, R. and Siddavattam, S. (2006) A novel meta-cleavage product hydrolase from Flavobacterium sp. ATCC27551. Biochem. Biophys. Res. Comm. 351: 675-681 (pdf file).

Pakala, S.B., Gorla, P., Pinjari, A.B., Krovidi, R.K., Baru, R., Yanamandra, M., Merrick, M. and Siddavattam, D.(2006) Biodegradation of methyl parathion and p-nitrophenol: Evidence for the presence of a p-nitrophenol 2–hydroxylase in a Gram-negative Serratia sp. strain DS001. App. Microbiol. Biotechnol. (pdf file).

Siddavatam, D., Raju, E.R., Emmanuel Paul, P.V. and Merrick, M. (2006) Overexpression of parathion hydrolase in Escherichia coli stimulates the synthesis of outer membrane porin OmpF. Pesticide Biochem. Physiol. 86: 146-150. (pdf file)

Manvathi, B., Pakala, S.B., Gorla, P., Merrick, M. and Siddavattam, D. (2005) Influence of zinc and cobalt on expression and activity of parathion hydrolase from Flavobacterium sp. ATCC27551. Pesticide Biochem. Physiol. 83: 37-45. (pdf file)

Siddavattam, D., Khajamohiddin, S., Manavathi, B., Pakala, S.B. and Merrick, M. (2003) Transposon-like organisation of the plasmid-borne organophosphate degradation (opd) gene cluster found in Flavobacterium sp. App. Env. Microbiol. 69: 2533-2539.(pdf file)

My Group

Group members

Dr Marta Radchenko
Project Scientist
marta.radchenko@bbsrc.ac.uk

Project: The role of protein complexes and protein localisation in regulation of bacterial nitrogen metabolism.

Mr Jeremy Thornton
Research Assistant
jeremy.thornton@bbsrc.ac.uk

Projects: Bacterial nitrogen metabolism and its regulation, the role of protein complexes and protein localisation in regulation of bacterial nitrogen metabolism and dissecting the nitrogen regulation system of streptomycetes.

Alumni

Former group members

	Dr Steve Pullan (2009) Steve.Pullan@nottingham.ac.uk
	Mr Alexandre Decorps (2009)
	Miss Juliana Inaba (2008)
	Mr Kolja Szymanski (2007-2008)
	Dr Luciano Huergo (2006) huergo@ufpr.br
	Dr Elizabeth Scanlon (2004-2007) elizabeth.scanlon@newcastle.ac.uk
	Dr Anne Durand (2004-2006) anne.durand@cgm.cnrs-gif.fr
	Dr Tim Fulford (2003-2006) t.fulford@surrey.ac.uk
	Dr Arnaud Javelle (2002-2005) http://www.lifesci.dundee.ac.uk/people/arnaud_javelle/
	Dr Emmanuele Severi (2002-2005) es455@hermes.cam.ac.uk
	Dr Graham Coutts (1999-2002)
	Dr Gavin Thomas (1998-2000) http://bioltfws1.york.ac.uk/biostaff/staffdetail.php?id=ght
	Dr Tania Arcondeguy (1997-2000) tania.arcondeguy@inserm.fr
	Dr Emma Southern (1996-1999)
	Dr Rachel Jack (1994-1998)
	Dr Rob Edwards (1991-1994) http://www.bio.sdsu.edu/faculty/faculty.html
	Dr Wyatt Paul (1986-1989) wyatt.paul@biogemma.com
	Dr Andreas Holtel (1984-1988)
	Dr Stuart MacFarlane (1983-1986) http://www.scri.ac.uk/staff/StuartMacFarlane
	Dr Ariel Álvarez Morales (1980-1983) http://www.iisd.ca/biodiv/bs-gflr/23%20feb.html
	Dr. Guadalupe Espin (1978-1981) http://www.ibt.unam.mx/server/PRG.base?tipo:doc,dir:PRG.curriculum,par:espin

Lab Publications since 1993

Li, X-D., Huergo, L.F., Gasperina, A., Pedrosa, F.O., Merrick, M. and Winkler, F.K. (2009) Crystal structure of dinitrogenase reductase activating glycohydrolase (DRAG) reveals conservation in the ADP-ribosylhydrolase fold and specific features in the ADP-ribose binding pocket. J. Mol. Biol. 390: 737-746 (pdf file).

Huergo, L.F., Merrick, M., Monteiro, R.A., Chubatsu, L.S., Steffens, M.B., Pedrosa, F.O. and Souza, E.M. (2009) In vitro interactions between the PII proteins and the nitrogenase regulatory enzymes DraT and DraG in Azospirillum brasilense. J Biol Chem. 284: 6674-6682 (pdf file).

Khajamohiddin, S., Repalle, E.J., Pinjari, A.B., Siddavattam, D. and Merrick, M. (2008) Biodegradation of aromatic compounds: An overview of meta-fission product hydrolases. Critical Reviews in Microbiology 34: 13-31.

Huergo, L.F., Merrick, M., Pedrosa, F.O., Chubatsu, L.S., Araujo, L.M. and Souza, E.M. (2007) Ternary complex formation between AmtB, GlnZ and the nitrogenase regulatory enzyme DraG reveals a novel facet of nitrogen regulation in bacteria. Mol. Microbiol. 66: 1523-1535. (pdf file)

Javelle, A., Lupo, D., Li, X-D., Merrick, M., Chami, M., Ripoche, P. and Winkler, F.K. (2007) Structural and mechanistic aspects of Amt/Rh proteins. J. Struct. Biol.158: 472-481 (pdf file)

Huergo L.F., Chubatsu L.S., Souza E.M., Pedrosa F.O., Steffens M.B.R. and Merrick M. (2006) Interactions between PII proteins and the nitrogenase regulatory enzymes DraT and DraG in Azospirillum brasilense. FEBS Letts. 580: 5232-5236 (pdf file)

Pakala S.B., Gorla P., Pinjari A.B., Krovidi R.K., Baru R., Yanamandra M., Merrick M. and Siddavattam D. (2006) Biodegradation of methyl parathion and p-nitrophenol: Evidence for the presence of a p-nitrophenol 2–hydroxylase in a Gram-negative Serratia sp. strain DS001. App. Microbiol. Biotechnol. (pdf file).

Conroy, M.J., Bullough, P.A., Merrick, M. and Avent, N.D. (2005) Modeling the human Rhesus proteins: implications for structure and function. Brit. J. Haematology 131: 534-551 (pdf file)

Javelle, A. and Merrick, M. (2005) Complex formation between AmtB and GlnK: an ancestral role in prokaryotic nitrogen control. Biochem. Soc. Trans. 33: 174-176. (pdf file)

Merrick, M.J. (2004) Nitrogen control of nitrogen fixation in free-living diazotrophs. In "Genetics and Regulation of Nitrogen Fixing Bacteria" eds: W. Klipp, B. Maepohl, J.R. Gallon and W.E. Newton. (pdf file)

von Wiren N. and Merrick, M. (2004) Regulation and function of ammonium carriers in bacteria, fungi and plants. Topics in Current Genetics 9: 95-120. (pdf file)

Siddavattam D., Khajamohiddin S., Manavathi B., Pakala S.B. and Merrick, M. (2003) Transposon-like organisation of the plasmid-borne organophosphate degradation (opd) gene cluster found in Flavobacterium sp. App. Env. Microbiol. 69: 2533-2539.(pdf file)

Blakey, D., Leech, A., Thomas, G.H., Coutts, G., Findlay, K. and Merrick, M. (2002) Purification of the Escherichia coli ammonium transporter AmtB reveals a trimeric stoichiometry. Biochem. J. 364: 527-535. (pdf file).

Arcondeguy, T., Jack, R. and Merrick, M. (2001) The P_II signal transduction proteins: pivotal players in microbial nitrogen control. Microbiol. Mol. Biol. Reviews 65: 80-105. (pdf file)

Ercolano, E., Mirabella, R., Merrick, M. and Chiurazzi, M. (2001) The Rhizobium leguminosarum glnB gene is down-regulated during symbiosis. Mol. Gen. Genet. 264: 555-564. (pdf file)

Thomas, G.H., Mullins, J. G. L. and Merrick, M. (2000) Membrane topology of the Mep/Amt family of ammonium transporters. Molecular Microbiology 37: 331-344. (pdf file)

Spinosa, M., Riccio, A., Mandrich, L., Manco, G., Lamberti, A., Iaccarino, M., Merrick M., and Patriarca, E.J. (2000) Inhibition of glutamine synthetase II expression by the product of the gstI gene. Molecular Microbiology 37: 443-452. (pdf file)

Arcondeguy, T., Lawson, D. and Merrick, M. (2000) Two residues in the T-loop of GlnK determine NifL-dependent nitrogen control of nifgene expression. J. Biol. Chem. 275:38452-38456. (pdf file).

Southern, E. and Merrick, M. (2000) The role of Region II in the RNA polymerase sigma factor sigma N (sigma 54). Nucleic Acids Research 28: 2563-2570. (pdf file)

Thomas, G., Coutts, G. and Merrick, M. (2000) The glnKamtB operon: a conserved gene pair in prokaryotes. Trends in Genetics 16: 11-14. (pdf file)

Arcondeguy, T., van Heeswijk, W.C., and Merrick, M. (1999). Studies on the roles of GlnK and GlnB in regulating Klebsiella pneumoniae NifL-dependent nitrogen control. FEMS Microbiol.Lett. 180: 263-270. (pdf file)

Taté, R., Cermola, M., Riccio, A., Iaccarino, M., Merrick, M., Favre, R. and Patriarca, E.J. (1999) The ectopic expression of the Rhizobium etli amtB gene affects the symbiosome differentiation process and nodule development. Molecular Plant-Microbe Interactions 12: 515-525. (pdf file)

Jack, R., de Zamaroczy, M. and Merrick, M. (1999) The signal transduction protein GlnK is required for NifL-dependent nitrogen control of nif gene expression in Klebsiella pneumoniae. J. Bacteriol 181: 1156-1162. (pdf file)

Taté, R., Riccio, A., Merrick, M. and Patriarca, E.J. (1998) The Rhizobium etli amtB gene coding for an NH4+ transporter is down-regulated early during bacteroid differentiation. Molecular Plant-Microbe Interactions 11: 188-198. (pdf file)

Juty, N.S., Moshiri, F., Merrick, M., Anthony, C. and Hill, S. (1997) The Klebsiella pneumoniae cytochrome bd' terminal oxidase complex and its role in microaerobic nitrogen fixation. Microbiol 143: 2673-2683. (pdf file)

Reizer, J., Reizer, A., Merrick, M.J., Plunkett, III G., Rose, D.J. and Saier, Jr. M.H. (1996) Novel phosphotransferase-encoding genes revealed by Analysis of the Escherichia coli genome: a chimeric gene encoding an Enzyme I homologue that possesses a putative sensory transduction domain. Gene 181: 103-108.

Spratt, B.G., Zhou, J., Taylor, M. and Merrick, M.J. (1996) Monofunctional biosynthetic peptidoglycan transglycosylases. Mol.Microbiol.19: 639-640.

Taylor, M., Butler, R., Chambers, S., Casimiro, M., Badii, F. and Merrick, M. (1996) The RpoN box motif of the RNA polymerase sigma factor sigmaN plays a role in promoter recognition. Mol.Microbiol. 22: 1045-1054.

Edwards, R. and Merrick, M. (1995) The role of uridylytransferase in the control of Klebsiella pneumoniae nif gene regulation. Mol.Gen.Genet. 247: 189-198.

Merrick, M.J. and Edwards, R.A. (1995) Nitrogen control in bacteria. Microbiol.Revs. 59: 604-622. (pdf file)

Merrick, M.J. and Taylor, M.S. (1994) Studies on the role of open-reading frames linked to the sigma factor gene rpoN in K.pneumonaiae. In Proceedings of the 1st European Nitrogen Fixation Conference (Edited by Kiss G.B. and Endre G.), p. 287. Officina Press, Szeged.

Merrick, M.J. (1994) Nitrogen fixation: regulation of gene expression. In The Encyclopedia of Molecular Biology (Edited by Kendrew J.), p. 731. Blackwell, Oxford.

Rieder, G., Merrick, M.J., Castorph, H. and Kleiner, D. (1994) Function of the hisF and hisH gene products in histidine biosynthesis. J.Biol.Chem. 269: 14386-14390.

Merrick, M.J. (1993) In a class of its own - the RNA polymerase sigma factor sigma54 (sigma N). Mol.Microbiol. 10: 903-909.