A RUBRERYTHRIN OPERON AND NIGERYTHRIN GENE IN DESULFOVIBRIO-VULGARIS (HILDENBOROUGH)

Rubrerythrin is a nonheme iron protein of unknown function isolated fr om Desulfovibrio vulgaris (Hildenborough). We have sequenced a 3.3-kbp Sa/I fragment of D. vulgaris chromosomal DNA containing the rubreryth rin gene, rbl, identified additional open reading frames (ORFs) adjace nt to rbr, and shown that these ORFs are part of a transcriptional uni t containing rbr. One ORF, designated fur, lies just upstream of rbr a nd encodes a 128-amino-acid-residue protein which shows homolog to Fur (ferric uptake regulatory) proteins from other purple bacteria. The o ther ORF, designated rdl, lies just downstream of rbr and encodes a 74 -residue protein with significant sequence homology to rubredoxins but with a different number and spacing of cysteine residues. Overexpress ion of rdl in Escherichia coli yielded a protein, Rdl, which has spect roscopic properties and iron content consistent with one Fe3+(SCys)(4) site per polypeptide but is clearly distinct from both rubrerythrin a nd a related protein, nigerythrin. Northern analysis indicated that fu r, rbr, and rdl were each present on a transcript of 1.3 kb; i.e., the se three genes are cotranscribed. Because D. vulgaris nigerythrin appe ars to be closely related to rubrerythrin, and its function is also un known, we cloned and sequenced the gene encoding nigerythrin, ngr. The amino acid sequence of nigerythrin is 33% identical to that of rubrer ythrin, and all residues which furnish iron ligands to both the FeS4 a nd diiron-oxo sites in rubrerythrin are conserved in nigerythrin. Desp ite the close resemblance of these two proteins, ngr was found to be n o closer than 7 kb to rbr on the D. vulgaris chromosome, and Northern analysis showed that, in contrast to rbr, ngr is not cotranscribed wit h other genes. Possible redox-linked functions for rubrerythrin and ni gerythrin in iron homeostasis are proposed.