A chromosomal cluster of genes encoding ADP-glucose synthetase, glycogen synthase and phosphoglucomutase in Agrobacterium tumefaciens.

A chromosomal region from Agrobacterium tumefaciens that complements exoC (pgm) mutations was cloned and sequenced. A cluster of three open reading frames (ORF1, ORF2 and ORF3) was identified. These genes are oriented in the same direction and are involved in the synthesis of glycogen and other polysaccharides. ORF1 encodes a 420-amino-acid (aa) protein with 55.9% homology to Escherichia coli GlgC (ADP-glucose synthetase, EC 2.7.7.27). ORF2 encodes a 480-aa protein with 42.2% homology to E. coli GlgA (glycogen synthase, EC 2.4.1.21). Based on Tn5 mutagenesis and protein homology, ORF3 was identified as the structural gene encoding phosphoglucomutase (Pgm, EC 2.7.5.1). ORF3 encodes a 542-aa protein with 52.6% homology to rabbit Pgm. There is no significant homology (less than 20%) to the Xanthomonas campestris XanA protein, which displays phosphomannomutase (Pmm) and Pgm activities [Koplin et al., J. Bacteriol 174 (1992) 191-199]. An A. tumefaciens pgm::Tn5 mutant retains Pmm activity.