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 σ
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 σ
70 DNA element located in the −10 region. However, instead of the σ
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 σ
70 with the −10 region. This type of activation, which is called ‘σ appropriation’, is fundamentally different from other well-characterized models of prokaryotic activation in which an activator frequently serves to force σ
70 to contact a less than ideal −35 DNA element. This review summarizes the interactions of AsiA and MotA with σ
70, and discusses how these interactions accomplish the switch to T4 middle promoters by inhibiting the typical contacts of the C-terminal region of σ
70, region 4, with the host −35 DNA element and with other subunits of polymerase.
Genetic Features of MCR-1-Producing Colistin-Resistant Escherichia coli Isolates in South Africa
