Transcriptional activation of the citrate permease P gene ofLactococcus lactis biovardiacetylactis by an insertion sequence-like element present in plasmid pCIT264

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Abstract
The lactococcal plasmid pCIT264 contains a cluster of three genes (citQ, citR andcitP) involved in the transport of citrate inLactococcus lactis biovardiacetylactis. Thecit cluster contains a copy of a newly discovered insertion sequence (IS)-like element located between its promoter P1 and the first gene of the cluster. In this report, we show that this IS-like element can act as a mobile switch for the downstream genes, creating two new transcriptional promoters named P2 and P2′. The P2 promoter is recognized by the lactococcal RNA polymerase in vivo. This is a hybrid promoter composed of a −35 region reading outwards 12 bp from the right end of the IS-like element, and a nucleotide sequence from the recipient plasmid, adjacent to the element, which provides an appropriately spaced −10 region. Transcription of thecitQRP cluster from this promoter takes place during the exponential and stationary phases of growth inL. lactis. Promoter P2′ is included in the IS-like element and is the only promoter responsible for expression ofcitP inE. coli. Thus, it appears that the introduction of this element into pCIT264 allows expression of thecitQRP cluster inE. coli, and increases its levels of expression inL. lactis.

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