C-type cytochromes in Pelobacter carbinolicus

Department of Microbiology, University of Massachusetts, Amherst, MA 01003

^* To whom correspondence should be addressed. Email: haveman{at}microbio.umass.edu,

	Abstract

Previous studies have failed to detect c-type cytochromes in Pelobacter species despite the fact that other close relatives in the Geobacteraceae, such as Geobacter and Desulfuromonas species, have abundant c-type cytochromes. Analysis of the recently completed genome sequence of Pelobacter carbinolicus revealed 14 open reading frames that could encode c-type cytochromes. Transcripts for all but one were detected in acetoin-fermenting and/or Fe(III)-reducing cells. Three putative c-type cytochrome genes were expressed specifically during Fe(III) reduction, suggesting that the encoded proteins may participate in electron transfer to Fe(III). One of these was a periplasmic triheme cytochrome with high similarity to PpcA, which has a role in Fe(III) reduction in Geobacter sulfurreducens. Genes for heme biosynthesis and system II cytochrome c biogenesis were identified in the genome and shown to be expressed. SDS-PAGE gels of protein extracted from acetoin-fermenting P. carbinolicus cells contained three heme-staining bands which were confirmed with mass spectrometry to be among the 14 predicted c-type cytochromes. The number of cytochrome genes, the predicted number of heme c per protein and the proportion of heme-stained to total protein was much smaller in P. carbinolicus compared with G. sulfurreducens. Furthermore, many of the c-type cytochromes that genetic studies have indicated are required for optimal Fe(III) reduction in G. sulfurreducens were not present in the P. carbinolicus genome. These results suggest that further evaluation of the function of c-type cytochromes in Geobacteraceae is warranted.