To examine the effects of sequence variations near the transcriptional start site on the rate of formation of the open complexes at bacteriophage $lambda P_{R}$ promoter, two mutant promoters were created by site-specific mutagenesis using synthetic oligonucleotides. Mutant I coatains changes at positions -3 and -4 from TT to CC, thus having a 6-bp long G/C stretch between -10 region and transciptional start site (+1). Mutant II has changes at positions -5 and -6 from GG to AA, thereby having a 9-bp long A/T stretch between positions -11 and -3. Selective filter binding assays were performed to measure the rate of formation of the open complexes between the wild-type or two mutant $P_{R}$ promoters on 664 bp fragments and E. coli RNA polymerase at two temperatures. At 37.deg.C, the wild-type and two mutants showed similar rates for the formation of open complex. The second order rate constant $k_{a}$ and $ au _{int}$, as determined from the .tau.-plot analysis, were $(6.0pm0.4) imes10^{6}M^{-1}sec^{-1}$ and $11pm5$sec, respectively. At 18.deg.C, however, the wild-type and two mutant promoters showed differences in the kinetic parameters. k for the wild-type promoter was (2.2$pm$0.1) imes 10^{6}M^{-1}sec^{-1}$</TEX> and $ au _{int}$ was 76$pm$sec. Mutant I and II exhibited differences mainly in the rate of isomerization ($ au_{int,I}=91pm$10 sec, int,II=34$pm$ sec), whereas the second order rate constant $k_{a}$ was similar to the wild type value. This result implies that at $18^{circ}C$, the isomerization rate is determined by both protein conformational change and DNA melting, which are separable kinetically according to the 3-step mechanism of Roe et al.(1984,1985), and that the base changes affected mainly the rate of DNA melting as predicted.lting as predicted.