ADP ribosylation of Escherichia coli RNA polymerase is nonessential for bacteriophage T4 development

The bacteriophage T4-induced alt and mod gene products covalently add ADP-ribose to the Escherichia coli RNA polymerase alpha polypeptides; phage carrying either an alt or a mod mutation are viable. A genetic cross between T4alt and T4mod phages yielded alt mod recombinant progeny which could not ADP ribosylate RNA polymerase at all, yet grew apparently normally. Thus, ADP ribosylation of RNA polymerase appeared to be nonessential for T4 development (at least in E. coli B/r and E. coli CR63), even though the phage has evolved two distinct enzymes to catalyze this reaction.