The dya5 mutation, a C→T change at position -43 of the λ pRE promoter, results in a twofold increase in pRE activity in vivo. Smaller increases in pRE activity are found for the dya2 mutation, a T→C change at position -1 of pRE, and the dya3 mutation, an A→G change at +5 of pRE. The mutant p RE promoters retain complete dependence on cII protein for activity. These observations argue, at least for pRE-like promoters, that promoter activities are influenced by nucleotide sequences at least eight nucleotides to the 5'-side of the conventional -35 region consensus sequence, and by nucleotide sequences near the start-site of transcription. Although Hawley and McClure (1983) found A·T pairs more frequently than G·TC pairs in the region of -40 to -45 of prokaryotic promoters, other mutations that change a G·TC pair to an A·T pair at positions -41, -44 and -45 of pRE do not result in increased promoter activity. We also found that a T→C change at position -42 results in a mild decrease in promoter activity. These observations argue that Ts at positions -42 and -43 of pRE are required for maximum promoter activity, but do not support the hypothesis that As and Ts in the -40 to -45 region generally lead to higher promoter activities.