cGMP production in bacteria.

Production of cGMP in bacteria has been studied since the early 1970s. From the beginning on it proved to be a challenging topic. In Escherichia coli the cGMP levels were two orders of magnitude lower than the corresponding cAMP levels. Furthermore, no specific cGMP receptor protein was identified in the bacterium and a physiological role of cGMP in the bacterium was not substantiated. Consequently in 1977, compelling evidence was given that cGMP is a by-product of E. coli adenylate cyclase in vivo. This may be the reason why also work on cGMP in other bacteria like Bacillus licheniformis and Caulobacter crescentus was not pursued any further. However, recent study on cGMP and guanylate cyclase in the cyanobacterium Synechocysis PCC 6803 brought cGMP signaling in bacteria back to attention. In Synechocystis cGMP levels are of similar magnitude as those of cAMP and deletion of the cya2 gene markedly reduced the amount of cGMP without affecting cAMP. A few months ago the Cya2 gene product has been biochemically and structurally characterized. It behaves as a specific guanylate cyclase in vitro and a single amino acid substitution transforms the enzyme into a specific adenylate cyclase. These data point toward the existence of a true bacterial cGMP-signaling pathway, which needs to be explored and established by future experiments.