Improved vector for promoter screening in lactococci.

Fragments of Lactococcus lactis subsp. lactis NP45 chromosomal DNA provided promoter activity in Escherichia coli when cloned into the promoter probe vector pGKV210. Only 13% of these recombinant plasmids promoted detectable cat-86 activity when transferred to L. lactis, i.e., expressed chloramphenicol resistance. In these promoter-containing versions of pGKV210, the cat-86 gene specifies chloramphenicol-inducible chloramphenicol acetyltransferase expression. This could be a limiting factor for cloning of promoters with lower activity in L. lactis. Therefore, we have constructed a new promoter probe vector, pBV5030, with the mutated version of the cat-86 gene, which is constitutively expressed when transcriptionally activated by the insertion of a promoter. We found that in L. lactis IL1403 the constitutively expressed cat-86 gene (on a pBV5030 derivative) has four times higher activity than the inducible version of the same gene (on a pGKV210 derivative) when both have the same promoter inserted upstream of the cat-86 gene. These results suggest that plasmid pBV5030 could be a more efficient vector for the cloning of promoters from lactococci.