The tyrosine decarboxylase operon of Lactobacillus brevis IOEB 9809: characterization and conservation in tyramine-producing bacteria.

Bacterial genes of tyrosine decarboxylases were recently identified. Here we continued the sequencing of the tyrosine decarboxylase locus of Lactobacillus brevis IOEB 9809 and determined a total of 7979 bp. The sequence contained four complete genes encoding a tyrosyl-tRNA synthetase, the tyrosine decarboxylase, a probable tyrosine permease and a Na+/H+ antiporter. Rapid amplification of cDNA ends (RACE) was employed to determine the 5'-end of mRNAs containing the tyrosine decarboxylase gene. It was located only 34-35 nucleotides upstream of the start codon, suggesting that the preceding tyrosyl-tRNA synthetase gene was transcribed separately. In contrast, reverse transcription-polymerase chain reactions (RT-PCRs) carried out with primers designed to amplify regions spanning gene junctions showed that some mRNAs contained the four genes. Homology searches revealed similar clusters of four genes in the genome sequences of Enterococcus faecalis and Enterococcus faecium. Phylogenetic analyses supported the hypothesis that these genes evolved all together. These data suggest that bacterial tyrosine decarboxylases are encoded in an operon containing four genes.