Regulation of the dsr operon and function of the proteins DsrR and DsrS in the purple sulfur bacterium <em>Allochromatium vinosum</em>

Abstract

This work represents the first in depth investigation of the dsr gene expression in a phototrophic sulfur-oxidizing organism. Although over the last decade some observations were made concerning regulation a more detailed survey had been missing until now.<br /> An increase in expression of dsr genes under sulfur-oxidizing conditions was shown on the transcriptional as well as translational level and probable secondary promoters involved in the expression of dsrC and dsrS were identified. Sulfide was disproven to be a direct inducing factor of the dsr gene expression, although the strength of the expression was dependent on the sulfide concentration. The oxidation of intermediary stored sulfur to sulfite was identified as the rate-limiting step in the oxidation of sulfur to sulfate. The proposed product of the reverse sulfite reductase sulfite as well as the alternative electron donor malate had no effect on the expression of dsrA. The proposed DNA-binding capability of DsrC could be confirmed, thus identifying DsrC as a potential regulatory protein of dsr gene expression.<br /> Furthermore, the importance of the previously barely investigated proteins DsrR and DsrS for the oxidation of intermediary stored sulfur in A. vinosum was examined more closely. The similarities and differences of DsrR to the A-type scaffold IscA, a protein involved in the maturation of iron-sulfur clusters, were shown on the structural as well as on the functional level and a potential direct involvement of DsrR in the maturation of Dsr protein specific iron-sulfur clusters could be negated. Interaction of DsrR with DsrEFH, DsrC, and DsrL opened up the possibility that DsrR might mediate the protein interaction of these Dsr proteins that could be involved in the substrate delivery system for the sulfite reductase. The effects of the deletion of dsrR and dsrS on the expression of other dsr genes were examined and, based on the results, a potential function for the proteins DsrR and DsrS as post-transcriptional regulators was proposed.