Evidence for a mosaic structure of the Tn5481 in Lactococcus lactis N8

The sequences of the left end of the nisin-sucrose transposon Tn5481 in Lactococcus lactis subsp. lactis N8, the adjacent 1.5 kb chromosomal region upstream of the junction site as well as a 5.0 kb region downstream of the nisZBTCIPRKFEG genes within the transposon have been determined. In the upstream chromosomal region, an incomplete open reading frame encoding a protein with strong N-terminal homology to the low-affinity branched chain amino acid carriers was identified. Within the transposon, downstream of the nisin gene cluster, a 186 bp almost identical copy of the left hand sequence was located. Further downstream, four new open reading frames were found. The codon usage in these reading frames as well as the G+C content of the region are clearly different from those of the nisin genes, suggesting that the functionally unrelated areas of Tn5481 are gathered from different origins during the evolution of the transposon.