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 op en reading frame, we generated an in-frame deletion allele via a gene replacement strategy. The resulting mutant grew significantly more slo wly than the wild type under lithoautotrophic conditions (6.1 versus 4 .2 h doubling time). A reduction in the level of the soluble NAD-reduc ing hydrogenase (60% of the wild-type activity) was shown to be the ca use of the slow lithoautotrophic growth. We used plasmid-borne gene fu sions to monitor the expression of the operons encoding the soluble an d membrane-bound hydrogenases. The expression of both operons nas lowe r in the mutant than in the wild-type strain. These results suggest th at the newly identified gene, designated hoxX, encodes a regulatory co mponent which, in conjunction with the transcriptional activator HoxA, controls hydrogenase synthesis.