The Alcaligenes eutrophus H16 hoxX gene participates in hydrogenase regulation

Nucleotide sequence analysis revealed a 1,791-bp open reading frame in the hox gene cluster of the gram-negative chemolithotroph Alcaligenes eutrophus H16. In order to investigate the biological role of this open reading frame, we generated an in-frame deletion allele via a gene replacement strategy. The resulting mutant grew significantly more slowly than the wild type under lithoautotrophic conditions (6.1 versus 4.2 h doubling time). A reduction in the level of the soluble NAD-reducing hydrogenase (60% of the wild-type activity) was shown to be the cause of the slow lithoautotrophic growth. We used plasmid-borne gene fusions to monitor the expression of the operons encoding the soluble and membrane-bound hydrogenases. The expression of both operons was lower in the mutant than in the wild-type strain. These results suggest that the newly identified gene, designated hoxX, encodes a regulatory component which, in conjunction with the transcriptional activator HoxA, controls hydrogenase synthesis.