SEQUENCE AND EXPRESSION OF THE MESSENGER-RNA ENCODING HSP22, THE MITOCHONDRIAL SMALL HEAT-SHOCK PROTEIN IN PEA LEAVES

A 3 h treatment at 40 degrees C of pea (Pisum sativum var. Douce Prove nce) plants induces production and accumulation of a small heat-shock protein of 22 kDa apparent molecular mass, designated HSP22, in the ma trix compartment of mitochondria [Lenne and Douce (1994) Plant Physiol . 105, 1255-1261]. We show her: that the HSP22 precursor (i.e. the mat ure protein plus the transit peptide) has an apparent molecular mass o f 26 kDa after in vitro translation of mRNA extracted from heat-stress ed pea plants and immunodetection. We have isolated, cloned and sequen ced the full-length cDNA encoding the precursor of the mitochondrial H SP22. An analysis of the amino acid sequence of the mitochondrial HSP2 2 reveals that this protein is a representative member of the low-mole cular-mass heat shock protein (HSP) superfamily, exhibiting the specif ic consensus regions that are typical of the small HSPs. Most importan tly, comparison of the mitochondrial HSP22 sequence with that of chlor oplast small HSPs indicates that HSP22 does not contain the typical ch loroplast consensus region III. We have also analysed the kinetics of HSP22 induction, and report results on the temporal expression of HSP2 2 at the transcriptional level. HSP22 mRNA was detected as soon as 10 min after the temperature was raised to a high temperature of 40 degre es C. Then the amount of HSP22 mRNA declined considerably even though pea plants were still submitted to the heat treatment. These results a re discussed in light of the translation data previously published [Le nne and Douce (1994) Plant Physiol. 105, 1255-1261], particularly conc erning the physiological behaviour of mitochondria when plants are hea t-stressed. Furthermore, we have studied the dependence of HSP22 accum ulation with temperature and demonstrate that the pea mitochondrial he at-shock response is only developed under extreme environmental growth conditions.