Predicted locations of the two candidate cysB genes in the genome of A. ferrooxidans. CysB is the proposed positive transcriptional regulator of the cys operon. A) CysB1 is found in a possible transcriptional unit with a candidate hoxA that we propose could be the positive transcriptional regulator of a gene cluster (hox1) that includes candidate genes for the hydrogenase small and large subunits and a number of auxiliary genes involved in hydrogenase formation (not shown). The cysB1/hoxA gene pair is adjacent to, but transcribed in the opposite orientation, to the predicted hox1 gene cluster. There is a second potential hydrogenase formation and expression gene cluster (hox2) but it is not physically connected with the hox1 gene cluster. (B) CysB2 is found in a possible transcriptional unit with a putative tcrr (transcriptional control response regulator) that has weak similarity to hoxA. Downstream, and possible in the same transcriptional unit are putative cobS and plr genes that are discussed in the text.