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. lact is N8 was cloned and sequenced. Analysis of the sequence revealed eigh t open reading frames, nisZBTC/PRK, followed by a putative rho-indepen dent terminator (Delta G degrees = -4.7 kcal/mol). The C-terminal hydr ophific domain of the NisK protein is homologous to the C-termini of s everal histidine kinases of bacterial two-component regulator systems, such as SpaK from Bariiius subtilis and KdpD and RcsC of Escherichia coli. The nisin Z biosynthetic genes were highly similar with the gene s of the nisin A operons having, however, a 0-3% difference in the ami no acid sequences of the individual proteins. The codon usage of eleve n genes within the same conjugative transposon was calculated and roun d to be strikingly different from that of other lactococcal genes. Thi s, together with the low GC-content (32%) compared to the 38% (G + C) of the lactococcai chromosome in general strongly suggests a non-lacto coccal origin of this transposon.