Developmental delay in a Streptomyces venezuelae glgE null mutant is associated with the accumulation of alpha-maltose 1-phosphate

The GlgE pathway is thought to be responsible for the conversion of trehalose into a glycogen-like alpha-glucan polymer in bacteria. Trehalose is first converted to maltose, which is phosphorylated by maltose kinase Pep2 to give alpha-maltose 1-phosphate. This is the donor substrate of the maltosyl transferase GlgE that is known to extend alpha-1,4-linked maltooligosaccharides, which are thought to be branched with alpha-1,6 linkages. The genome of Streptomyces venezuelae contains all the genes coding for the GlgE pathway enzymes but none of those 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 Delta 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 of those of the wild-type, implying impaired cross-wall formation, and spores showed reduced tolerance to stress. The mutant accumulated alpha-maltose 1-phosphate and maltose but no alpha-glucan. Therefore, the GlgE pathway is necessary and sufficient for polymer biosynthesis. Growth of the Delta glgE mutant on galactose and that of a Delta pep2 mutant on maltose were analysed. In both cases, neither accumulation of alpha-maltose 1-phosphate/alpha-glucan nor a developmental delay was observed. Thus, high levels of alpha-maltose 1-phosphate are responsible for the developmental phenotype of the Delta glgE mutant, rather than the lack of a-glucan.