Phosphorylation of Escherichia coli translation initiation factors by the bacteriophage T7 protein kinase

The lytic coliphage T7 encodes a serine/threonine-specific protein kinase which supports viral reproduction under suboptimal growth conditions. Expression of the protein kinase in T7-infected Escherichia coli cells results in the phosphorylation of 30S ribosomal subunit protein S1, and initiation factors IF1, IF2, and IF3, as determined by high-resolution two-dimensional gel electrophoresis and specific immunoprecipitation analysis. Phosphorylation occurs either exclusively on threonine (IF1, IF3, S1) or on serine and threonine (IF2). There is no phosphorylation of these proteins in uninfected cells or in cells infected with T7 lacking the protein kinase function. Phosphorylation of the initiation factors coincides with the onset of T7 late protein synthesis, occurring 9-12-min postinfection. T7 late protein synthesis, otherwise inhibited in ColIb plasmid-containing cells, is specifically supported by expression of the protein kinase. These results provide the first evidence for the functional involvement of protein phosphorylation in the control of bacterial translation.