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Microbiology 151 (2005), 1761-1767; DOI  10.1099/mic.0.27870-0
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Microbiology 151 (2005), 1761-1767; DOI  10.1099/mic.0.27870-0
© 2005 Society for General Microbiology

Regulation of two highly similar genes, omcB and omcC, in a 10 kb chromosomal duplication in Geobacter sulfurreducens

Ching Leang and Derek R. Lovley

Department of Microbiology, 203 Morrill Science Center IVN, 639 North Pleasant St, University of Massachusetts at Amherst, Amherst, MA 01003, USA

Correspondence
Ching Leang
leang{at}microbio.umass.edu

The Fe(III)-reducing micro-organism Geobacter sulfurreducens requires an outer-membrane c-type cytochrome, OmcB, for Fe(III) reduction, but a related cytochrome, OmcC, which is 73 % identical to OmcB, is not required. The omcB and omcC genes are part of a tandem chromosomal duplication consisting of two repeated clusters of four genes. The 2·7 kb sequences preceding omcB and omcC are identical with the exception of a single base pair change. Studies that combined genetic, Northern blotting and primer extension analyses demonstrated that both omcB and omcC are transcribed as monocistronic and polycistronic (orf1-orf2-omcB/omcC) transcripts. All of the promoters for the various transcripts were found to be located within the 2·7 kb identical region upstream of omcB and omcC. The sequences of the promoter regions for the two monocistronic transcripts are identical and equidistant from the omcB or omcC start codons. The promoters for the two polycistronic transcripts, in contrast, are distinct. One is specific for transcription of orf1-orf2-omcB and the other is associated with transcription of orf1-orf2-omcC. Studies with an RpoS-deficient mutant suggested that transcription from all four promoters is RpoS dependent under one or more growth conditions. Deletion of orfR, a gene immediately upstream of orf1-orf2-omcB that encodes a putative transcriptional regulator, significantly lowered the omcB transcription when Fe(III) was the electron acceptor and partially inhibited Fe(III) reduction. In contrast, levels of omcC transcripts were unaffected in the orfR mutant. These results indicate that omcB and omcC operons represent a rare instance in which duplicated operons, located in tandem on the chromosome, have different transcriptional regulation.

The GenBank/EMBL/DDBJ accession numbers for the sequences reported in this paper are as follows. orfR, AAR36113 The omcB operon: orf1, AAR36111 orf2, AAR36110 omcB, AAR36109 orfS, AAR36107 The omcC operon: orf1, AAR36105 orf2, AAR36104 omcC, AAR36103




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