Association of the prophage P1ban protein with the dnaB protein of Escherichia coli.

The molecular mechanism by which Escherichia coli dnaB mutations are suppressed by prophage Plbac has been investigated. The prophage Pl codes for a dnaB analog (ban) protein which is repressed in cells containing Pl wild type and expressed constitutively in cells containing the mutant Plbac. Plbac ban mutants fail to synthesize an active ban protein (D’Ari, R., Jaff&Brachet, A., Touati-Schwartz, D., and Yarmolinsky, M. B. (1975). J. Mol. Biol. 94, 341-366; Ogawa, T. (1975) J. Mol. Biol. 94, 327-340). An enzyme has been isolated from Plbac lysogenic E. coli strains which complements an extract of a thermosensitive (ts) E. coli dnaB mutant assayed for in vitro DNA synthesis. Lysogens of a dnaB amber mutant are used as enzyme source in which the amber mutation is either unsuppressed (su-) or suppressed by the suppressors supE or supF resulting in a thermosensitive and thermoresistant dnaB protein, respectively (Ogawa, T. (1975) J. Mol. Biol. 94, 327-340). Highly purified enzyme preparations from all strains tested revealed native molecular weights for the enzyme of approximately 260,000 as estimated by glycerol gradient centrifugation. The enzyme isolated from Plbac lysogens of the E. coli supE and supF strains is composed of polypeptides with molecular weights of 62,000 + 56,000 and 61,000 + 56,000, respectively. The larger components (M, = 62,000 and M, = 61,000) are presumably the dnaB monomers synthesized by supE and supF strains. The 56,000 dalton polypeptide is absent in nonlysogens, Pl wild type, and Plbac ban lysogens, and it is the only component of the 260,000 dalton enzyme found in the purified preparation from a dnaB su(Plbac) lysogen. It has therefore been tentatively identified as the Pl ban protein. In Plbac lysogens dnaB and ban monomers are assumed to be associated with one another in heteromultimers. The enzymatic activity of such heteromultimers isolated from the supE(Plbac) lysogens is thermolabile in contrast to the thermostable heteromultimers from supF(Plbac) lysogens. It appears that suppression of the supE dnaB ts mutation by Plbac results from a stabilization of the dnaB ts polypeptide by ban subunits in a heteromultimer. The heteromultimers contain a DNA-dependent and -independent ribonucleoside triphosphatase activity. Both the dnaB complementing and the DNA-dependent ribonucleoside triphosphatase activity are inactivated by antibodies directed against dnaB.