Evidence is presented here which indicates that the adenovirus DNA-binding protein (DBP) is phosphorylated at a tyrosine residue early in infection. This was suggested by the discovery that a proportion of the label in P-labelled DBP was resistant to alkali, and was substantiated by acid hydrolysis of DBP immunoprecipitates and by immunoblotting with a monoclonal antibody against phosphotyrosine. Treatment of [S] methionine-labelled DBPs with chymotrypsin produced fragments of apparent 45K and 39K whereas digestion of P-labelled DBP resulted in fragments of 45K and 26K. Consideration of the distribution of P label and its alkali stability in these fragments suggested that chymotrypsin cleaved populations of DBP at different sites depending on their phosphorylation states. The conservation, in all of the seven adenovirus serotypes sequenced, of a tyrosine residue (at amino acid 195 in adenovirus type 2) together with its surrounding residues, suggests that phosphorylation/dephosphorylation at this tyrosine residue may be important in various functions ascribed to the DBP.


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