L-Hydroxyproline and D-Proline Catabolism in Sinorhizobium meliloti

Abstract

Hydroxyproline as a modified amino acid can serve as a carbon and nitrogen source for certain microorganisms. Its primary isomer trans-4-hydroxy-L-proline is found in the root nodule of legume plants. Hydroxyproline (Hyp) catabolism has been characterized in bacteria and animal cells. In bacteria, trans-4-hydroxy-L-proline (trans-4-L-proline) is converted to the central metabolite α-ketoglutarate (α-KG) by four reactions. The Hyp catabolism pathway has been identified in the nitrogen-fixing legume endosymbiont Sinorhizobium meliloti. hypS is one of the transcripts in the 14 hyp gene cluster on the pSymB megaplasmid, and was annotated to encode a putative malate/L-lactate dehydrogenase. In this study, purified HypS was assayed on different substrates and the reaction products were characterized. It was demonstrated that HypS can oxidize L-proline and reduce Δ1-pyrroline-2-carboxylate, but not on L-malate. Noticeably unlike the wild type strain, a hypS- mutant strain failed to grow on D-proline. The ability of D-proline to support grow of an L-proline auxotroph, together with the substrate specificity of HypS, strongly suggests that hypS is involved in the metabolism of D-proline to L-proline in S. meliloti. The possible role of HypS in the catabolism of Hyp or related compounds remains to be determined.