| Lysine acetylation is a posttranslational modification that occurs on the epsilon-amine group of lysine residues and can affect enzyme activity, interactions with other proteins, and protein stability. Relatively little is known about lysine acetylation in bacteria, although proteomics studies indicate that it may be a common posttranslational modification. This work describes what we have learned about lysine acetylation in the purple photosynthetic bacterium Rhodopseudomonas palustris, a metabolically versatile organism found in soil and fresh water.;R. palustris has a lysine posttranslational modification system that includes two acetyltransferase enzymes, RpPat and RpKatA, and two deacetylase enzymes, RpSrtN and RpLdaA. Initial experiments showed that this system regulates the activities of enzymes that function in the catabolism of aromatic acids. Specifically, enzymes belonging to the AMP-forming acyl-CoA synthetase family that activate aromatic acids to acyl-CoA thioesters were inactivated by acetylation of a catalytic lysine residue, and could be reactivated by deacetylation. Tandem mass spectrometry was used to identify additional substrates of this acetylation system, which included nine AMP-forming acyl-CoA synthetases used to activate a variety of fatty acids. Targeted studies have now shown that at least 16 acyl-CoA synthetases are regulated by acetylation in R. palustris. As many of these fatty acid degradation pathways ultimately yield acetyl-CoA, which also serves as the substrate for RpPat and RpKatA, we hypothesize that acetylation may serve as a form of feedback inhibition for acyl-CoA synthetase enzymes.;RpPat, the better studied of the two acetyltransferases, appears to specifically acetylate acyl-CoA synthetases at a conserved, catalytic lysine residue. The protein sequence surrounding the acetylation site is important for acetylation by RpPat, and minor changes can abolish acetylation. These residues make up a structural loop that is presumably recognized by RpPat. In addition, a second structural loop of acyl-CoA synthetases, located >20 A away, is also important for recognition by RpPat; at least one acyl-CoA synthetase, MatB, has changes in this loop that prevent it from being acetylated by RpPat. These results show that RpPat specifically acetylates a subset of acyl-CoA synthetases in R. palustris. |
