Induction of the histidine decarboxylase genes of Photobacterium damselae subsp. damselae (formally P. histaminum) at low pH.

AIMS

To elucidate the detailed mechanism of histamine production by Photobacterium damselae subsp. damselae.

METHODS AND RESULTS

Histidine decarboxylase and related genes of P. damselae subsp. damselae were cloned, and three open reading frames named as hdcT, hdcA and hisRS were identified. The hdcA gene encodes a polypeptide of 377 amino acids and is considered to be the pyridoxal-P dependent histidine decarboxylase. The hdcT gene is assumed to be a histidine/histamine antiporter, and the hisRS gene is considered to be a histidyl-tRNA synthetase. Recombinant Escherichia coli strains harbouring plasmids carrying the P. damselae hdc genes were shown to over-excrete histamine extracellularly. Northern blot analysis and quantitative RT-PCR revealed high levels of mono- and bi-cistronic transcripts of hdcA, hdcT and hisRS genes under conditions of low pH and histidine excess.

CONCLUSIONS

The hdcA gene of P. damselae was constructed as an operon with putative histidine/histamine antiporter and histidyl-tRNA synthetase. Mono- and poly-cistronic transcripts and acid induction were detected.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first report of cloning the histidine decarboxylase gene cluster in gram-negative bacteria. Also, these genes were induced under acidic conditions and in the presence of excess histidine.