Transcriptional analysis of nitrogen fixation in Paenibacillus durus during growth in nitrogen-enriched medium

Paenibacillus durus strain ATCC 35681T is a Gram-positive diazotroph that displayed capability of fixing nitrogen even in the presence of nitrate or ammonium. However, the nitrogen fixation activity was detected only at day 1 of growth when cultured in liquid nitrogen-enriched medium. The transcripts of all the nifH homologues were present throughout the 9-day study. When grown in nitrogen-deficient medium, nitrogenase activities occurred from day 1 until day 6 and the nifH transcripts were also present during the course of the study albeit at different levels. In both studies, the absence of nitrogen fixation activity regardless of the presence of the nifH transcripts raised the possibility of a post-transcriptional or post-translational regulation of the system. A putative SigA box sequence was found upstream of the transcription start site of nifB1, the first gene in the major nitrogen fixation cluster. The upstream region of nifB2 showed a promoter recognisable by SigE, a sigma factor normally involved in sporulation. Significance and Impact of the StudyPaenibacillus durus strain ATCC 35681T is a nitrogen fixing Gram positive bacterium with an unconventional physiological characteristic of being able to fix nitrogen even in the presence of either nitrate or ammonium. It has a total of 6 nifH homologues in its genome. In this study, we analysed the transcriptional levels of the nifH homologues when grown under nitrogen-enriched and nitrogen-depleted medium. Under nitrogen-enriched condition, the nitrogen fixation activity was detected only at day 1 of growth but the transcripts of all the nifH homologues were detected during the course of the study from day 1 until day 9. In nitrogen-deficient condition, nitrogen fixation activities were recorded from day 1 until day 6 and the nifH transcripts were present throughout the study. The absence of nitrogen fixation activity even in the presence of the nifH transcripts raised the possibility of a post-transcriptional or post-translational regulation of the system.