hupS and hupL constitute a transcription unit in the cyanobacterium Nostoc sp. PCC 73102.

The heterocystous cyanobacterium Nostoc sp. PCC 73102 possesses an uptake hydrogenase encoded by hupS and hupL. The genes are transcribed in cells grown under N2-fixing but not under non-N2-fixing conditions. We characterised the transcription of hupS and hupL in further detail. A transcription start site was located at a position 259 bp upstream from the hupS translation start. As shown by RT-PCR, hupS and hupL constitute a single transcript in Nostoc sp. PCC 73102. The possible function of short, tandemly repeated repetitive sequences between hupS and hupL, enabling a folding of the entire intergenic region into a hairpin, is discussed.