Exposure of maize seedlings to a rapid elevation in temperature (heat shock, hs; 25-42.5 C) results in the synthesis of heat shock polypeptides (hsp's) with Mr's of 108, 89, 84, 73 (hereafter referred to as 70) and 18kd. These hsp's are detected in vivo and are prominent among the in vitro translation products of polyribosomal poly(A)+ RNAs from heat-shocked plumules.

Northern hybridization analyses suggest that the hs-enhanced synthesis of the 70kd hsp results from an increase in the accumulation of 70kd RNA transcripts on both free-cytoplasmic and membrane/cytoskeletal-associated ribosomes. In contrast, RNA blot analyses indicate that synthesis of the more prominent 18kd hsp variants results from the new transcription of 18kd RNA transcripts and the subsequent accumulation of these transcripts on free-cytoplasmic and membrane/cytoskeletal-associated ribosomes. In each case, this response occurs after a 5 to 10 minute exposure to hs and is maximal after 1-2h of hs.

Continued exposure to hs or a return of maize seedlings to 25 C (after a 1h hs) results in a decline in both the 70kd and 18kd hs mRNAs associated with these fractions. The kinetics of hs-induction and recovery are reflected in both the in vivo and in vitro (cell-free translation of polyribosomal poly(A)+ RNAs) protein synthesis profiles. Although hybridization studies (using a maize hsp 70 and a maize hsp 18 cDNA) indicate discrete variances in both the time and degree of accumulation of 70kd and 18kd hsp mRNAs between the free-cytoplasmic and membrane/cytoskeletal-associated ribosomal fractions, these variances are not significant. However, the induction and recovery of the 70kd and 18kd hsp mRNAs are non-coordinate.

N.C. Hogan*, D.B. Walden and B.G. Atkinson*

*Department of Zoology

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