PHYLOGENY OF DISSIMILATORY SULFITE REDUCTASES SUPPORTS AN EARLY ORIGIN OF SULFATE RESPIRATION

Microorganisms that use sulfate as a terminal electron acceptor for an aerobic respiration play a central role in the global sulfur cycle. He re, we report the results of comparative sequence analysis of dissimil atory sulfite reductase (DSR) genes from closely and distantly related sulfate-reducing organisms to infer the evolutionary history of DSR, A 1.9-kb DNA region encoding most of the alpha and beta subunits of DS R could be recovered only from organisms capable of dissimilatory sulf ate reduction with a PCR primer set targeting highly conserved regions in these genes, All DNA sequences obtained were highly similar to one another (49 to 89% identity), and their inferred evolutionary relatio nships were nearly identical to those inferred on the basis of 16S rRN A, We conclude that the high similarity of bacterial and archaeal DSRs reflects their common origin from a conserved DSR, This ancestral DSR was either present before the split between the domains Bacteria, Arc haea, and Eucarya or laterally transferred between Bacteria and Archae a soon after domain divergence. Thus, if the physiological role of the DSR was constant over time, then early ancestors of Bacteria and Arch aea already possessed a key enzyme of sulfate and sulfite respiration.