Ribulose 1,5-Bisphosphate Carboxylase Synthesis during Heat Shock

Ribulose 1,5-bisphosphate carboxylase (RuBPCase) was chosen as a model protein to study how heat shock (HS) affects both chloroplast protein synthesis and the nuclear-chloroplast interaction in production of chloroplast proteins. Experiments were performed using highly chlorophyllous, soybean (Glycine max L. Merr. var Corsoy) cell suspension cultures active in chloroplast protein synthesis. Synthesis of RuBPCase large (L) and small (S) subunits was followed by in vivo labeling, and corresponding mRNA levels were examined by Northern and dot hybridization analyses. Results demonstrate that L and S synthesis declines with increasing HS temperatures (33-40 degrees C) and reaches minimum levels (20-30% of control) at temperatures of maximum HS protein synthesis (39-40 degrees C). Recovery of L and S synthesis following a 2-hour HS at 38 or 40 degrees C was also studied. The changes in S synthesis during HS and recovery correlate with the steady state levels of S mRNA. In contrast, changes in L synthesis show little relationship to the corresponding mRNA levels; levels of L mRNA remain relatively unchanged by HS. We conclude that chloroplast protein synthesis shows no greater sensitivity to HS than is observed for cytoplasmic protein synthesis and that transport of proteins into the chloroplast (e.g.S subunit) continues during HS. Furthermore, there is no apparent coordination of L and S subunit mRNA levels under the conditions examined.