The mac promoters: functional hybrid promoters activated by the malT product and repressed by the lacI product.

Using in vitro techniques we have fused upstream sequences from the malPp promoter (normally activated by the MalT protein) to downstream sequences from the lacZp promoter (normally repressed by the LacI protein). Several hybrid promoters were thus obtained, which were controlled by the MalT protein, but were poorly active. More efficient promoters were then isolated using in vivo selection. Three main conclusions could be derived from the analysis of all of these hybrid promoters. Firstly, the MalT protein seems able to force RNA polymerase to start transcription at any DNA sequence, albeit with a low efficiency. Secondly, the strength of the hybrid promoters is considerably increased if a Pribnow Box is positioned at a precise location with respect to the MalT binding site. Thirdly, the presence of the lac operator, even when properly positioned with respect to the transcription startpoint, does not suffice to permit full repression by the lacI product.