Hybrid PLtl promoter with dual regulation control.

The newly constructed PLtl hybrid promoter is composed of the operator and promoter sequences of tac and the -35 to -135 region of the phage lambda PL promoter, which contains the AT-rich block and the OL2 and OL3 segments. Transcriptional properties of PLtl were examined and compared with tac and lambda PL as reference promoters. The hybrid PLtl exhibits different and improved properties over tac promoter in four ways: (i) when repressed, the repression is almost complete; (ii) after complete induction, the hybrid PLtl promoter shows a 1.4-2 times higher expression; (iii) the hybrid PLtl promoter permits flexible gene expression because it can be utilized under either or both repression controls simultaneously; (iv) the PLtl promoter permits enhanced expression of genes encoding products with unknown properties. When compared with the strong promoter PL from phage lambda, results with the PLtl promoter in lacZ fusion constructs show higher levels of beta-galactosidase activity. We also constructed a plasmid vector, pPLtl7G, which contains the hybrid PLtl promoter with a polylinker sequence at its 3' end, which facilitates efficient fusions of foreign genes in any of the reading frame.