Fig. 5. The activity of the TTF1 mutants still depends on PKA. Wild-type TTF1 or the mutants described in Fig. 4 were coexpressed in A126 cells with pTgTTF1-CAT in the presence () or absence ({blacksquare}) of PKA expression vectors. Mutant threonine (9) -> alanine alters a TTF1 residue reported to be phosphorylated by PKA in lung (16) . The results are expressed as CAT enzymatic activity and represent means of four independent experiments that yielded similar results; bars, SD.