Activation of bacteriophage T4 middle promoters, which occurs about 1 min after infection, uses two phage-encoded factors that change the promoter specificity of the host RNA polymerase. These phage factors, the MotA activator and the AsiA co-activator, interact with the sigma(70) specificity subunit of Escherichia coli RNA polymerase, which normally contacts the -10 and -35 regions of host promoter DNA. Like host promoters, T4 middle promoters have a good match to the canonical sigma(70) DNA element located in the -10 region. However, instead of the sigma(70) DNA recognition element in the promoter's -35 region, they have a 9 bp sequence (a MotA box) centred at -30, which is bound by MotA. Recent work has begun to provide information about the MotA/AsiA system at a detailed molecular level. Accumulated evidence suggests that the presence of MotA and AsiA reconfigures protein-DNA contacts in the upstream promoter sequences, without significantly affecting the contacts of sigma(70) with the -10 region. This type of activation, which is called 'sigma appropriation', is fundamentally different from other well-characterized models of prokaryotic activation in which an activator frequently serves to force sigma(70) to contact a less than ideal -35 DNA element. This review summarizes the interactions of AsiA and MotA with sigma(70), and discusses how these interactions accomplish the switch to T4 middle promoters by inhibiting the typical contacts of the C-terminal region of sigma(70), region 4, with the host -35 DNA element and with other subunits of polymerase.