The effect of a bacteriophage T4-induced polypeptide on host RNA polymerase interaction with promoters.

After infection of Escherichia coli with bacteriophage T4, the host RNA polymerase acquires several small phage-induced polypeptides (Stevens, A. (1974) Biochemistry 13, 493-503) and its alpha subunits get ADP-ribosylated by a virus-specific enzyme (Zillig, W., Mailhammer, R., Skorko, R., and Rohrer, H. (1977) Curr. Top. Cell. Regul. 12, 263-271). The modified polymerase displays changed enzymatic properties including sensitivity to increased salt concentration and a higher transition temperature of open promoter complex formation (promoter melting temperature). In order to assess the role of individual modifications in the changed enzyme properties, we isolated RNA polymerase from cells infected with T4 mutant defective in the ADP-ribosylating enzyme. We also purified one of the associated polypeptides, the 15,000-dalton protein which is invariably present in stoichiometric amounts in different RNA polymerase preparations. In an in vitro transcription system using T4 DNA as template, we demonstrate that the 15-kDa protein is the cause of the elevated promoter melting temperature and can induce this property when added to host RNA polymerase. We also show that the increased salt sensitivity of T4-modified polymerase is primarily the result of ADP-ribosylation of its alpha subunits.