A HISTIDINE-DECARBOXYLASE GENE ENCODED BY THE VIBRIO-ANGUILLARUM PLASMID PJM1 IS ESSENTIAL FOR VIRULENCE - HISTAMINE IS A PRECURSOR IN THE BIOSYNTHESIS OF ANGUIBACTIN

We have identified and sequenced an hdc gene in the Vibrio anguillarum plasmid pJM1 which encodes a histidine decarboxylase enzyme and is an essential component for the biosynthesis of anguibactin. The open rea ding frame corresponds to a protein of 386 amino acids with a calculat ed molecular mass of 44 259.69 Da. The amino acid sequence has extensi ve homoIogy with the pyridoxal-P-dependent histidine decarboxylases of Morganelia morganii, Klebsiella planticola, and Enterobacfer aerogene s. Tn3-HoHo1 transposition mutagenesis of the hde gene present in a re combinant clone carrying the entire pJM1 iron uptake region produced t wo derivatives, one with the lacZ gene in the same orientation as the direction of hde transcription and the other with the lacZ gene in the opposite orientation. A V.anguillarum strain harbouring one of the mu tated derivatives was unable to grow under iron-limiting conditions an d did not produce anguibactin, Therefore, the hdc gene must play a rol e in the biosynthetic pathway of this siderophore and consequently in conferring the high virulence phenotype to this bacterium. The role of histidine decarboxylase in biosynthesis of anguibactin was confirmed by the fact that growth under iron starvation was restored by addition of histamine to the medium. The presence of anguibactin was also demo nstrated in supernatants from cultures of the hdc mutant strains grown under iron starvation with the addition of histamine, further confirm ing that histamine is a precursor in the biosynthesis of the sideropho re. Immunoblot analysis of production of beta-galactosidase by V. angu illstum strains carrying the lacZ fusions demonstrated that expression of histidine decarboxylase is not regulated by the iron concentration of the medium.