Translational control of bacteriophage MS2 RNA cistrons by MS2 coat protein: affinity and specificity of the interaction of MS2 coat protein with MS2 RNA

The effect of MS2 coat protein on MS2-specific protein synthesis was studied in vitro.Protein synthesis was initiated by adding MS2 RNA to the in vitro protein-synthesizing system and MS2 coat protein was added to the system at varying time intervals afterwards. The addition of coat protein specifically reduced the rate of viral RNA polymerase synthesis (assayed by histidine incorporation) without significantly affecting the rate of coat protein synthesis (assayed by phenylalanine incorporation). The addition of coat protein to the on-going system rapidly reduced histidine incorporation to a rate comparable to that which occurred when coat protein was incubated with MS2 RNA (to form “complex I”) prior to their addition to the protein-synthesizing system. The concentrations of MS2 RNA and MS2 coat protein used in one experiment were quite low and probably in the range of their actual concentrations in the cytoplasm of the infected host cells.These results indicate (1) that the interaction of MS2 coat protein and MS2 RNA is highly specific, since various cellular components present in the protein-synthesizing system did not interfere with the effect of the added coat protein and (2) that the affinity of coat protein for MS2 RNA is high enough for the formation of complex I to take place at the relatively low concentrations of MS2 RNA and coat protein presumed to occur in the cytoplasm of the infected cells.