Mechanisms of acetate formation and acetate activation in halophilic archaea

The halophilic archaea Halococcus (Hc.) saccharolyticus, Haloferax (Hf.) vo lcanii, and Halorubrum (Hr.) saccharovorum were found to generate acetate d uring growth on glucose and to utilize acetate as a growth substrate. The m echanisms of acetate formation from acetyl-CoA and of acetate activation to acetyl-CoA were studied. Nc. saccharolyticus, exponentially growing on com plex medium with glucose, formed acetate and contained ADP-forming acetyl-C oA synthetase (ADP-ACS) rather than acetate kinase and phosphate acetyltran sferase or AMP-forming acetyl-CoA synthetase. In the stationary phase, the excreted acetate was completely consumed, and cells contained AMP-forming a cetyl-CoA synthetase (AMP-ACS) and a significantly reduced ADP-ACS activity . Nc. saccharolyticus, grown on acetate as carbon and energy source, contai ned only AMP-ACS rather than ADP-ACS or acetate kinase. Cell suspensions of Nc. saccharolyticus metabolized acetate only when they contained AMP-ACS a ctivity, i.e., when they were obtained after growth on acetate or from the stationary phase after growth on glucose. Suspensions of exponential glucos e grown cells, containing only ADP-ACS but not AMP-ACS, did not consume ace tate. Similar results were obtained for the phylogenetic distantly related halophilic archaea Hf. volcanii and Hf. saccharovorum. We conclude that, in halophilic archaea, the formation of acetate from acetyl-CoA is catalyzed by ADP-ACS, whereas the activation of acetate to acetyl-CoA is mediated by an inducible AMP-ACS.