The GlgE pathway is thought to be responsible for the conversion of trehalose into a glycogen-like a-glucan polymer in bacteria. Trehalose is first converted into maltose, which is phosphorylated by maltose kinase Pep2 to give a-maltose 1-phosphate. This is the donor substrate of the maltosyl transferase GlgE that is known to extend a-1,4 linked maltooligosaccharides, which are thought to be branched with a-1,6 linkages. The genome of Streptomyces venezuelae contains all of the genes coding for the GlgE pathway enzymes but none of related pathways, including glgC and glgA of the glycogen pathway. This provides an opportunity to study the GlgE pathway in isolation. The genes of the GlgE pathway were upregulated at the onset of sporulation, consistent with the known timing of a-glucan deposition. A constructed ?glgE null mutant strain was viable but showed a delayed developmental phenotype when grown on maltose, giving less cell mass and delayed sporulation. Pre-spore cells and spores of the mutant were frequently double the length, implying impaired cross-wall formation, and spores showed reduced tolerance to stress. The mutant accumulated a-maltose 1-phosphate and maltose but no a-glucan. Therefore, the GlgE pathway is necessary and sufficient for polymer biosynthesis. The growth of the ?glgE mutant on galactose and that of a ?pep2 mutant on maltose were analysed. In both cases, there was neither the accumulation of a-maltose 1-phosphate/a-glucan nor a developmental delay. Thus, high levels of a-maltose 1-phosphate are responsible for the developmental phenotype of the ?glgE mutant, rather than the lack of a-glucan.