Analysis of plasmids encoding the tyrosine decarboxylase gene in Tetragenococcus halophilus isolated from fish sauce

In order to elucidate mechanisms of tyramine accumulation during fish sauce production, two tyramine-producing bacterial strains, referred to as TyrA and TyrB, were isolated from fish sauce mash accumulating over 141 mg of tyramine per 100 g of sample. Both strains were identified as Tetragenococcus halophilus based on phenotypic characterization and a 16S rRNA gene sequence analysis. Molecular analysis of the tyramine-producing gene in the two strains confirmed the presence of a ~30-kb plasmid encoding a single copy of the pyridoxal phosphate-dependent tyrosine decarboxylase gene ( tdcA ) along with three other genes related to tyramine synthesis ( tdc cluster). The complete nucleotide sequences of plasmids extracted from the two strains indicated that both plasmids were almost identical, except for a 1.6-kb transposon sequence in the plasmid from the strain TyrB. Both plasmids had a replication region, a plasmid maintenance region, and two putative mobile genetic elements located upstream and downstream of the tdc cluster. This structure was identical to that of tetragenococcal plasmids encoding histidine decarboxylase ( hdcA ), which were sequenced previously. These results suggest a common origin for plasmids encoding hdcA and tdcA . In addition, the genes for both these biogenic amines are distributed among tetragenococcal species via this plasmid.