The transcriptional regulators RamA and RamB are involved in the regulation of glycogen synthesis in Corynebacterium glutamicum

When grown in glucose-, fructose- or sucrose-containing medium, the amino acid producer Corynebacterium glutamicum transiently accumulates large amounts of glycogen (up to 10% of its dry weight), whereas only a marginal amount of glycogen is formed during growth with acetate. This carbon-source-dependent regulation is at least partially due to transcriptional control of glgC, encoding ADP-glucose pyrophosphorylase, the first enzyme of glycogen synthesis from glucose-1-phosphate. Here, we have analysed a possible regulatory role for the transcriptional regulators RamA and RamB on glycogen content of the cells and on control of expression of glgC and of glgA, which encodes the second enzyme of glycogen synthesis, glycogen synthase. Determination of the glycogen content of RamA- and RamB-deficient C. glutamicum indicated that RamA and RamB influence glycogen synthesis positively and negatively, respectively. In accordance with the identification of putative RamA and RamB binding sites upstream of glgC and glgA, both regulators were found to bind specifically to the glgC-glgA intergenic promoter region. Promoter activity assays in wild-type and RamA- and RamB-deficient strains of C. glutamicum revealed that (i) RamA is a positive regulator of glgC and glgA, (ii) RamB is a negative regulator of glgA and (iii) neither RamA nor RamB alone is responsible for the carbon-source-dependent regulation of glycogen synthesis in C. glutamicum.