Characterization of the cycHJKL genes involved in cytochrome c biogenesis and symbiotic nitrogen fixation in Rhizobium leguminosarum.

Mutants of Rhizobium leguminosarum bv. viciae unable to respire via the cytochrome aa3 pathway were identified by the inability to oxidize N,N'-dimethyl-p-phenylenediamine. Two mutants which were complemented by cosmid pIJ1942 from an R. leguminosarum clone bank were identified. Although pea nodules induced by these mutants contained many bacteroids, no symbiotic nitrogen fixation was detected. Heme staining of cellular proteins revealed that all cytochrome c-type heme proteins were absent. These mutants lacked spectroscopically detectable cytochrome c, but cytochromes aa3 and d were present, the latter at a higher-than-normal level. DNA sequence analysis of complementing plasmids revealed four apparently cotranscribed open reading frames (cycH, cycJ, cycK, and cycL). CycH, CycJ, CycK, and CycL are homologous to Bradyrhizobium japonicum and Rhizobium meliloti proteins thought to be involved in the attachment of heme to cytochrome c apoproteins; CycK and CycL are also homologous to the Rhodobacter capsulatus ccl1 and ccl2 gene products and the Escherichia coli nrfE and nrfF gene products involved in the assembly of c-type cytochromes. The absence of cytochrome c heme proteins in these R. leguminosarum mutants is consistent with the view that the cycHJKL operon could be involved in the attachment of heme to apocytochrome c.