Different roles for basic and aromatic amino acids in conserved region 2 of Escherichia coli sigma(70) in the nucleation and maintenance of the single-stranded DNA bubble in open RNA polymerase-promoter complexes.

Amino acid residues in region 2 of final sigma(70) have been shown to play an important role in the strand separation step that is necessary for formation of the functional or open RNA polymerase-promoter complex. Here we present a comparison of the roles of basic and aromatic amino acids in the accomplishment of this process, using RNA polymerase bearing alanine substitutions for both types of amino acids in region 2. We determined the effects of the substitutions on the kinetics of open complex formation, as well as on the ability of the RNA polymerase to form complexes with single-stranded DNA, and with forked DNA duplexes carrying a single-stranded overhang consisting of bases in the -10 region. We concluded that two basic amino acids (Lys(414) and Lys(418)) are important for promoter binding and demonstrated distinct roles, at a subsequent step, for two aromatic amino acids (Tyr(430) and Trp(433)). It is likely that these four amino acids, which are close to each other in the structure of final sigma(70), together are involved in the nucleation of the strand separation process.