SummaryThe mesophilic methanogenic archaeon Methanosarcina mazei strain Gö1 is able to utilize molecular nitrogen (N 2 ) as its sole nitrogen source. We have identified and characterized a single nitrogen fixation (nif ) gene cluster in M. mazei Gö1 with an approximate length of 9 kbp. Sequence analysis revealed seven genes with sequence similarities to nifH, nifI 1 , nifI 2 , nifD, nifK, nifE and nif N, similar to other diazotrophic methanogens and certain bacteria such as Clostridium acetobutylicum, with the two glnB-like genes (nifI 1 and nifI 2 ) located between nifH and nifD. Phylogenetic analysis of deduced amino acid sequences for the nitrogenase structural genes of M. mazei Gö1 showed that they are most closely related to Methanosarcina barkeri nif 2 genes, and also closely resemble those for the corresponding nif products of the grampositive bacterium C. acetobutylicum. Northern blot analysis and reverse transcription PCR analysis demonstrated that the M. mazei nif genes constitute an operon transcribed only under nitrogen starvation as a single 8 kb transcript. Sequence analysis revealed a palindromic sequence at the transcriptional start site in front of the M. mazei nifH gene, which may have a function in transcriptional regulation of the nif operon.