The nitrogen fixation (nif) genes of Rhodopseudomonas capsulata appear to be much more widely dispersed than those of the well-studied Klebsiella pneumoniae. In both organisms the genes coding for the structural components of the nitrogenase complex, nifHDK, comprise a single unit of transcription. Only in the photosynthetic bacterium, however, are there multiple copies of these genes. The extra copies are normally silent but they can be activated by mutation, detected as nif+ pseudorevertants of nif- deletions of the expressed nifHDK genes. Glutamine is required for the repression of nif gene transcription in R. capsulata.