The amplification of an internal fragment of the hdcA gene for histidine decarboxylase in Lactobacillus buchneri showed the gene to be located on the bacterial chromosome. Reverse PCR was then used to amplify both it and its adjacent genes. The histidine decarboxylase cluster was found to be composed of four genes: hdcC (expressed in Lactococcus lactis, the product of which is located in the membrane, suggesting it to be a histidine/histamine antiporter), hdcA (which encodes histidine decarboxylase), hdcB (of unknown function but co-transcribed as bicistronic mRNA together with hdcA) and hisS (the only copy of a gene encoding a histidyl-tRNA synthetase in Lb. buchneri). The expression of hisS depends on the histidine concentration of the growth medium, and it can be transcribed as monocistronic or hdcA-hdcB-hisS polycistronic mRNA.