Diurnal regulation of Hsp70s in leaf tissue.

Steady-state mRNA levels for three Hsp70s were found to be regulated by a distinctive light/dark mechanism in spinach leaves. Messenger RNAs for the chloroplast stromal and two cytosolic forms displayed a diurnal expression pattern under isothermal conditions that appeared to be independent of circadian control. While protein blot data showed relatively constant Hsp70 protein levels, the higher Hsp70 mRNA levels in the light paralleled the diurnal cycle of total cell protein synthesis. Fractionation studies showed that the major cytosolic Hsp70 cognate group was associated with polysomes. Therefore, the variation of Hsp70 mRNAs is consistent with the diurnal metabolic activity of plant photosynthetic cells in which the demand of protein biogenesis for chaperone function and tissue temperature are highest during the day.