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.     

Temporally regulated tandem promoters in Micromonospora echinospora

A collection of promoters from the Micromonospora echinospora strain that produces the calichemicin antitumor antibiotics was identified by the use of the promoter-probe vector pIJ486 in Streptomyces lividans. A 0.4-kilobase-pair Micromonospora DNA fragment was found to contain multiple tandem promoters which were characterized by S1 nuclease protection, Northern blotting, and DNA sequence determination. Analysis of RNA isolated from timed Micromonospora cultures revealed two classes of promoters within the 0.4-kilobase-pair fragment. The P2 promoter was maximally active during the exponential phase. In contrast, the P1 promoter cluster, consisting of three closely spaced start sites located 80 base pairs upstream of P2, was maximally active during the stationary phase. Because P1 was strongly induced in synchrony with calichemicin drug production, P1 is of potential utility in expressing cloned genes specifically during the stationary phase.