From the nifc mutant plasmid pPC868, previously shown to carry a DNA duplication responsible for the Nifc phenotype, a 10 kb HindIII fragment was cloned into the multicopy vector pBR325. Restriction analysis of the resulting plasmids and in vitro deleted derivatives confirmed that the mutation was a fusion between his and nifLA. The order was hisG-hisD'-'nifL-nifA so that nifA was transcribed under the control of the his promoter and the nifL gene was altered. In addition the cloned fragment contained the adjacent nifBQ operon, and complementation data revealed that the nifA, nifB and hisG genes were expressed. Synthesis of nifA product under the transcription control of the his (or cat [CmR]) promoter enabled complementation of nifA and nifB mutations either in the absence or the presence of ammonia, but did not restore nitrogen fixation in a glnF mutant. Therefore, the nifA gene product requires glnF for its positive control function in a manner analogous to ntrC. Protein content analysis of minicells containing various multicopy nif plasmids confirmed the genetic organization mentioned above. A new polypeptide of 51,500 daltons was found whose synthesis was observed at 30 degrees C but not at 37 degrees C. According to the physical map, this protein could be the nifB gene product. Our results are in agreement with nifB transcription being under the control of a thermolabile nifA product. Moreover we obtained results suggesting that the presence of multiple copies of a functional nifB gene inhibited nitrogen fixation.