Expression of the adenyl cyclase-encoding gene (cya) of Rhizobium meliloti F34: existence of two cya genes?

Cloning of the second adenylate cyclase gene (cya2) from Rhizobium meliloti F34: sequence similarity to eukaryotic cyclases

A second adenylate cyclase (cya2) gene was isolated from a Rhizobium meliloti F34 gene bank. Complemented E. coli delta cya mutants were capable of utilizing a number of, but not all, carbon sources known to be regulated by cAMP. DNA hybridization studies showed cya2 to be unique to R. meliloti strains. The cya2 nucleotide sequence was determined and found to encode a protein of 363 amino acids. Residues were identified within the C-terminal domain which are conserved in both eukaryotic adenylate and guanylate cyclases, including a putative ATP binding site. Similar residues were also found in the prokaryotic R. meliloti Cya1 protein. A R. meliloti cya1/cya2 double mutant was constructed and characterized; however, cAMP production was still observed in this strain indicating the presence of a third cya gene.

Cyclic AMP in prokaryotes

Cyclic AMP (cAMP) is found in a variety of prokaryotes including both eubacteria and archaebacteria. cAMP plays a role in regulating gene expression, not only for the classic inducible catabolic operons, but also for other categories. In the enteric coliforms, the effects of cAMP on gene expression are mediated through its interaction with and allosteric modification of a cAMP-binding protein (CRP). The CRP-cAMP complex subsequently binds specific DNA sequences and either activates or inhibits transcription depending upon the positioning of the complex relative to the promoter. Enteric coliforms have provided a model to explore the mechanisms involved in controlling adenylate cyclase activity, in regulating adenylate cyclase synthesis, and in performing detailed examinations of CRP-cAMP complex-regulated gene expression. This review summarizes recent work focused on elucidating the molecular mechanisms of CRP-cAMP complex-mediated processes. For other bacteria, less detail is known. cAMP has been implicated in regulating antibiotic production, phototrophic growth, and pathogenesis. A role for cAMP has been suggested in nitrogen fixation. Often the only data that support cAMP involvement in these processes includes cAMP measurement, detection of the enzymes involved in cAMP metabolism, or observed effects of high concentrations of the nucleotide on cell growth.

Rhizobium meliloti adenylate cyclase is related to eucaryotic adenylate and guanylate cyclases

A gene from Rhizobium meliloti coding for an adenylate cyclase was sequenced, and the deduced protein sequence was compared with those of other known adenylate cyclases. No similarity could be detected with the procaryotic counterparts. However, striking similarity was found with the catalytic region of Saccharomyces cerevisiae adenylate cyclase, the cytoplasmic domains of bovine adenylate cyclase, and two mammalian guanylate cyclases. The gene was fused to the enteric beta-galactosidase, and the chimeric protein was purified by affinity chromatography. This fusion protein was found to direct the synthesis of cyclic AMP in vitro. This activity was strongly inhibited by the presence of GTP, but no cyclic GMP synthesis could be detected in conditions permitting cyclic AMP synthesis.