Identification of bidirectional hydrogenase genes and their co-transcription in unicellular halotolerant cyanobacterium Aphanothece halophytica

Abstract

The halotolerant cyanobacterium Aphanothece halophytica has been shown to produce H2 via dark fermentation of accumulated glycogen under anoxic condition. One set of hox genes encoding a bidirectional hydrogenase is present in A. halophytica. In this study, the nucleotide sequence and the transcriptional analysis of hox genes in A. halophytica were investigated. The results revealed that A. halophytica contained five structural genes, hoxE, hoxF, hoxU, hoxY, and hoxH, without an insertion of other open reading frames (ORFs). The conserved cysteine motifs of iron-sulfur clusters involved in an electron transfer were found in all Hox subunits. The nucleotide and deduced amino acid sequences of hox genes in A. halophytica showed the highest identity and similarity to those of Halothece sp. PCC 7418. By reverse transcription polymerase chain reaction (RT-PCR) analysis, hox genes in A. halophytica were co-transcribed as a single operon. Under nitrogen-deprived condition, the transcripts of hoxH, glgB, coxA, ndhB, and psaA were upregulated whereas those of glgP and narB were downregulated which resulted in an increase of H2 production, H2ase activity, glycogen content, and dark respiration rate.