Competition between sigma factors for core RNA polymerase.

The switch of RNA polymerase specificity from early to late promoters of bacteriophage T4 is achieved by substitution of host sigma factor, sigma 70, with the T4 induced factor, sigma gp55. However, overproduction of sigma gp55 from an expression vector is not detrimental to Escherichia coli growth. Direct competition binding assays demonstrate that sigma 70 readily displaces sigma gp55 from RNA polymerase and thereby reverses the promoter specificity of the enzyme. The displacement also occurs with the core enzyme modified by bacteriophage T4 infection. We postulate that an antagonist of sigma 70 should be formed in T4-infected cells to aid sigma gp55 in the early/late switch.