Control of bacteriophage phi X 174 gene expression by the transcription termination factor rho in vitro.

The effect of rho factor on bacteriophage phi X 174 gene expression was studied in a cell-free, DNA-dependent, RNA-directed protein synthesizing system (transcription-translation coupled system) using S30 extracts of wild-type and rho-mutant Escherichia coli cells. It has been found that in the presence of a functionally active and endogenous or added rho factor, only a limited number of the viral genes (A, B, and D) are expressed effectively, whereas the expression of genes, F, G, and H, which encode capsid proteins, is strongly inhibited. Mutational or thermal inactivation of rho factor results in a considerable activation of capsid protein synthesis and, at the same time, in some depression of the synthesis of gene A, B, and D products. From the results of this study it is suggested that phage phi X 174 has two classes of gene, late and early, which are separated on the chromosome by a rho-dependent transcription terminator. We propose that the expression of these classes is regulated by a termination-antitermination mechanism.