The glnAntrBC operon of Proteus vulgaris was cloned and heterologously expressed in Escherichia coli. The nucleotide sequence was determined. An open reading frame of 1407 bp was identified as the glnA gene and the deduced amino acid sequence showed 82% identity with the E. coli glutamine synthetase protein. Heterologous expression of the glnA gene in E. coli restored glutamine synthetase (GS) activity in a GS-negative mutant and a 52 kDa protein was detected and addressed as the GS subunit of P. vulgaris. Adjacent to the glnA gene the regulatory genes ntrB and ntrC were identified. Their coding regions comprised 1053 and 1452 bp, respectively, and the deduced gene products NRII (NtrB) and NRI (NtrC) shared 72% identity with the corresponding E. coli proteins. Heterologous expression in E. coli revealed only a 54 kDa protein which was shown to be NRI. NRII was not detectable using the methods employed.