In Vivo Expression of lac Promoter Variants with Altered -10, -35, and Spacer Sequences

The structural elements that constitute an Escherichia coli promoter have been studied intensively during the last few years. Most promoter mutants map to the mod-erately conserved "-10 " and "-35 " promoter regions (for review, see Siebenlist et al. 1980), implicating these two elements in promoter function in vivo. Recently, we have identified a third element that is im-portant for promoter function in vitro (Stefano and Gralla 1982a). We termed this element he spacer because it separates the-10 and-35 region sequences; the number of base pairs in the spacer was shown to be an important determinant of promoter function in vitro (Stefano and Gralla 1982a). The extrapolation of the im-portance of this element to an in vivo situation is sup-ported by the existence ofdown mutations in the spacer