Dual transcriptional control of the acetaldehyde dehydrogenase gene ald of Corynebacterium glutamicum by RamA and RamB.

Corynebacterium glutamicum has been shown to grow with ethanol as the sole or as additional carbon and energy source and accordingly, to possess both alcohol dehydrogenase and acetaldehyde dehydrogenase (ALDH) activities, which are responsible for the two-step ethanol oxidation to acetate. Here we identify and functionally analyze the C. glutamicum ALDH gene (cg3096, ald), its expression and its regulation. Directed inactivation of the chromosomal ald gene led to the absence of detectable ALDH activity and to the inability to grow on or to utilize ethanol, indicating that the ald gene product is essential for ethanol metabolism and that no ALDH isoenzymes are present in C. glutamicum. Transcriptional analysis revealed that ald from C. glutamicum is monocistronic, that ald transcription is initiated 92 nucleotides upstream of the translational start codon ATG and that ald expression is much lower in the presence of glucose in the growth medium. Further analysis revealed that transcription of the ald gene is under control of the transcriptional regulators RamA and RamB. Both these proteins directly bind to the respective promoter region, RamA is essential for expression and RamB exerts a slightly negative effect on ald expression on all carbon sources tested.