Abstract
Ribosomes isolated from Drosophila melanogaster tissue culture cells labeled in vivo with 32Pi contain a single, heavily phosphorylated, ribosomal protein. As much as 40% of this protein is phosphorylated in cells cultured at 25 degrees C. The molecular weight and other characteristics of this protein suggest possible homology with ribosomal protein S6. Following a shift-up to 37 degrees C, the protein is specifically and quantitatively dephosphorylated. The kinetics of this dephosphorylation are rapid with a half-time on the order of a few minutes. These kinetics closely parallel the heat shock-induced breakdown of the preexisting polysome population.