Heat-shock-induced alterations of ribosomal protein phosphorylation in plant cell cultures

Heat shock of cell suspension cultures of tomato (Lycopersicon peruvianum) results in a rapid and reversible decline of the phosphorylation level of a single basic ribosomal protein of the small subunit (tentatively identified as ribosomal protein S6). Simultaneously, phosphate labeling of several acidic ribosomal proteins of the large subunit is enhanced. Data on the temperature-dependent distribution of S6 subspecies and on the kinetics and reversibility of S6 phosphorylation are given. The decreased phosphorylation of S6 at temperatures higher than 35 degrees C coincides with the onset of heat shock protein synthesis and precedes a decline of the mitotic index. Recovery from heat shock is characterized by S6 rephosphorylation and, subsequently, leads to an abnormally high mitotic index.