Diurnal regulation of Hsp70s in leaf tissue

Steady-state mRNA levels for three Hsp70s were found to be regulated by a d istinctive light/dark mechanism in spinach leaves. Messenger RNAs for the c hloroplast stromal and two cytosolic forms displayed a diurnal expression p attern under isothermal conditions that appeared to be independent of circa dian control. While protein blot data showed relatively constant Hsp70 prot ein levels, the higher Hsp70 mRNA levels in the light paralleled the diurna l cycle of total cell protein synthesis. Fractionation studies showed that the major cytosolic Hsp70 cognate group was associated with polysomes. Ther efore, the variation of Hsp70 mRNAs is consistent with the diurnal metaboli c activity of plant photosynthetic cells in which the demand of protein bio genesis for chaperone function and tissue temperature are highest during th e day.