Cloning and transcriptional analysis of the nifUHDK genes of Trichodesmiumsp. IMS101 reveals stable nifD, nifDK and nifK transcripts

Trichodesmium spp. are marine filamentous, non-heterocystous cyanobacteria capable of aerobic nitrogen fixation. In this study, the nitrogenase struct ural genes (nifHDK) and nifU gene of Trichodesmium sp. IMS101 were cloned a nd sequenced. The Trichodesmium sp. IMS101 nifH, nifD and nifK amino acid s equences showed only 79%, 66% and 68% identity, respectively, to those of A nabaena sp. strain PCC 7120. A potential transcription start site for nifH was found 212 bases upstream of the nifH start codon. Promoter-like nucleot ide sequences upstream of the transcription start site were identified that were very similar to those identified for the nitrogenase genes of Anabaen a spp. Sequence analysis revealed regions of DNA that may form stem-loop st ructures in the intercistronic regions downstream of nifH and nifD. RNA ana lysis by Northern hybridization revealed the presence of transcripts corres ponding to nifH, nifHD and nifHDK. Surprisingly, Northern hybridization als o revealed the presence of transcripts that corresponded to nifD, nifDK and nifK, which have not been previously reported as transcripts in contiguous nifHDK genes of cyanobacteria. Transcription of the nifHDK genes was not s ignificantly repressed in the presence of nitrate at a final concentration of 20 mM or at oxygen concentrations of up to 40%, whereas ammonium and ure a inhibited nifHDK transcription. The transcription of the nifHDK genes was not affected by darkness, which suggests that transcription of these genes in Trichodesmium is not directly regulated by light.