Lyngbya majuscula CCAP 1446/4 is a filamentous cyanobacterium possessing both an uptake and a bi-directional hydrogenase. The presence of a single copy of the hyp operon in the cyanobacterial genomes suggests that these accessory genes might be responsible for the maturation of both hydrogenases. We investigated the concomitant transcription of hypFCDEAB with the hydrogenases structural genes--hup and hox. RT-PCRs performed with L. majuscula cells grown under different physiological conditions showed a substantial decrease in the relative amount of hupL transcript under non-N2-fixing conditions. In contrast, no significant differences were observed for the transcript levels of hypFCDEAB in all conditions tested, while minor fluctuations could be discerned for hoxH. Previously, it was demonstrated that the transcriptional regulators NtcA and LexA interact with the promoter regions of hup and hox, respectively, and that putative binding sites for both proteins are present in the hyp promoter of L. majuscula. Therefore, a putative involvement of NtcA and LexA in the regulation of the hyp transcription was investigated. Electrophoretic mobility shift assays resulted in NtcA or LexA-bound retarded fragments, suggesting the involvement of these proteins in the transcriptional regulation of hypFCDEAB.