Effect of bacteriophage M13 infection on phosphorylation of dnaK protein and other Escherichia coli proteins

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Abstract
1. The effects of infection with the filamentous phage M13 on the phosphorylation of Escherichia coli proteins were studied. Phosphorylated proteins were labeled with [32P]orthophosphate and analyzed by the O''Farrell two-dimensional gel technique and autoradiography. 2. Phage infection was shown to induce significant changes in the pattern of protein phosphorylation. At least eight different proteins were found to be phosphorylated to a larger extent while seven others were, by contrast, much less labeled than in uninfected bacteria. 3. Labeling experiments with [35S]methionine demonstrated that these quantitative changes in protein phosphorylation were not connected, in any case, with changes in the amount of protein synthesized. They rather seemed to result from a variation of the phosphorylating capacity of the relevant protein kinase(s). 4. The individual proteins, whose phosphorylation was affectd by phage infection, were characterized by both their molecular mass and isoelectric point. One of them, whose phosphorylation was increased by a factor of 7, was identified as the dnaK protein which is necessary for both cellular and phage DNA replication. 5. The chemical analysis of the phosphorylated moiety of dnaK protein showed that is was modified exclusively at serine residues during normal growth of cells, and mostly at threonine residues after phage infection. These results were discussed in terms of stimulation of the protein activity by phosphorylation.