The codon usage of the nisZ operon in Lactococcus lactis N8 suggests a non-lactococcal origin of the conjugative nisin-sucrose transposon.

An 11.6 kb area downstream from the structural gene of nisin Z in the conjugative nisin-sucrose transposon of Lactococcus lactis subsp. lactis N8 was cloned and sequenced. Analysis of the sequence revealed eight open reading frames, nisZBTClPRK, followed by a putative rho-independent terminator (delta G degrees = -4.7 kcal/mol). The C-terminal hydrophilic domain of the NisK protein is homologous to the C-termini of several histidine kinases of bacterial two-component regulator systems, such as SpaK from Bacillus subtilis and KdpD and RcsC of Escherichia coli. The nisin Z biosynthetic genes were highly similar with the genes of the nisin A operons having, however, a 0-3% difference in the amino acid sequences of the individual proteins. The codon usage of eleven genes within the same conjugative transposon was calculated and found to be strikingly different from that of other lactococcal genes. This, together with the low GC-content (32%) compared to the 38% (G+C) of the lactococcal chromosome in general strongly suggests a non-lactococcal origin of this transposon.