Bacterial promoters of the extended -10 class contain a single consensus element, and the DNA sequence upstream of this element is not critical for promoter activity. Open promoter complexes can be formed on an extended -10 Escherichia coli galP1 promoter at temperatures as low as 6 °C, when complexes on most promoters are closed. Here, we studied the contribution of upstream contacts to promoter complex formation using galP1 and its derivatives lacking the extended -10 motif and/or containing the -35 promoter consensus element. A panel of E. coli RNA polymerase holoenzymes containing two, one, or no a-subunit C-terminal domains (αCTD) and either wild-type σ70 subunit or σ70 lacking region 4.2 was assembled and tested for promoter complex formation. At 37 °C, αCTD and σ70 region 4.2 were individually dispensable for promoter complex formation on galP1 derivatives with extended -10 motif. However, no promoter complexes formed when both αCTD and σ70 region 4.2 were absent. Thus, in the context of an extended -10 promoter, αCTD and σ70 region 4.2 interactions with upstream DNA can functionally substitute for each other. In contrast, at low temperature, αCTD and σ70 region 4.2 interactions with upstream DNA were found to be functionally distinct, for σ70 region 4.2 but not αCTD was required for open promoter complex formation on galP1 derivatives with extended -10 motif. We propose a model involving σ70 region 4.2 interaction with the β flap domain that explains these observations.