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 affected 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 it 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.