POLY(A) TAIL LENGTH OF A HEAT-SHOCK PROTEIN RNA IS INCREASED BY SEVERE HEAT-STRESS, BUT INTRON SPLICING IS UNAFFECTED

The small heat shock proteins (sHSPs) are induced in all eukaryotes in response to high temperature stress, but are most abundant among memb ers of the plant kingdom where they accumulate in multiple subcellular compartments. We have analyzed the expression of the chloroplast-loca lized sHSP from Arabidopsis thaliana, HSP21, and characterized the str ucture of the gene encoding this protein to facilitate future genetic studies on the function of HSP21 in the heat shock response. HSP21 is encoded in Arabidopsis by a single gene whose coding region is interru pted by a single intron. Previous studies have shown that intron proce ssing is disrupted by severe, abrupt heat stress but is protected by p retreatments that induce thermotolerance. The processing of the HSP21 transcript was investigated in response to an abrupt heat stress regim e and a gradual heat stress regime, the latter of which is known to co nfer thermotolerance in plants. Under abrupt stress conditions the HSP 21 transcript is somewhat longer than under gradual heat stress condit ions. However, the molecular basis for the size difference is not impa ired intron splicing, but rather a difference in the length of the pol y(A) tail depending on the heat stress regime. The results suggest tha t an increase in poly(A) tail length may be a generalized response to severe, abrupt heat stress and that poly(A) tail metabolism may be one of numerous cellular processes normally protected in thermotolerant c ells from the otherwise damaging effects of high temperature stress.