Nitrogenase messenger RNA synthesis in Klebsiella pneumoniae was determined by labelling cells with (3H)uracil and isolating total RNA, which was then hybridized to filter-bound recombinant plasmid pSA30 DNA carrying the nitrogenase structural genes nifH, D, and K, Derepression of nitrogenase mRNA starts 1.5 h before the onset of nitrogenase activity (as measured by acetylene reduction). Exposure of nif-derepressed cultures to either NH4+, air, or high temperatures (39 degree C) results in a rapid decrease of the synthesis rates both of nitrogenase mRNA and nitrogenase polypeptides. Nitrogenase mRNA is remarkably stable. After blocking transcription with rifampicin, hybridizable and actively translatable nitrogenase mRNA survives with an average half-life of 18 min. Half-lives are considerably shorter when rifampicin-inhibited cultures are simultaneously shifted to conditions which are non-permissive for nitrogenase synthesis, pointing to some posttranscriptional influence on nitrogenase mRNA stability. In all experiments performed there was no evidence for uncoupling of nitrogenase mRNA synthesis from nitrogenase mRNA translation, indicating that nitrogenase synthesis is regulated solely by transcriptional control.