In an attempt to throw further light on the relationship of R17 phage RNA and A-protein during the early stages of infection, studies were carried out to determine the effect of ribonuclease (ribonuclease I, EC 3.1.4.22) on the ability of these two phage components to penetrate into host bacteria. It was found that the penetration of phage RNA is affected by ribonuclease concentrations as low as 0.1 mug/ml, while the penetration of phage A-protein was unaffected by ribonuclease concentrations as high as 20 mug/ml. In addition, it was found that a significant fraction of the phage RNA is resistant to the ribonuclease effect. This RNase-resistant portion of the phage population increased with increasing phage concentrations, and gave rise to the penetration of intact, 28S RNA molecules that produced the expected number of infectious centers. These findings are discussed in terms of a model for phage RNA injection in which the A-protein functions both as an attachment organelle and a pilot protein that guides the RNA from the capsid to the exterior surface of the F pilus, and thence into the host bacterium.
Coliphages Endonucleases