Date of Award


Degree Type


Degree Name

Doctor of Philosophy


Translational control of protein synthesis in the plumules of heat-shocked maize seedlings was investigated. Maize plumules respond to heat shock by synthesizing nuclear-encoded hsp's with M{dollar}\sb{lcub}\rm r{rcub}{dollar}'s of 108, 89, 84, 76, 73, and 17-29kDa. Synthesis of some, but not all, 25{dollar}\sp\circ{dollar}C (control) proteins is repressed in vivo. RNA blot and in vitro translation analyses of non-polyribosomal and polyribosomal RNP RNA suggest that repression ensues from either a decrease in mRNA levels or from the inefficient translation of control messages in heat-shocked cells (relative to heat shock mRNAs) due to changes in the rates of both initiation and elongation.;Hsp70 and hsp18 mRNAs (or heat shock-like mRNAs) are synthesized at low levels in nonstressed cells. During heat shock, the messages accumulate on polyribosomes within 5 to 10 minutes, are maximal within 1 to 2h and decline thereafter (ie. maize plumules acclimate). The recovery profile is similar. Dissociation of message from the polyribosomes under these conditions is not accompanied by a re-association of heat shock mRNAs with non-polyribosomal RNPs suggesting that (1) the messages released from the polyribosomes are degraded and (2) the mechanism(s) governing these responses is independent of the incubation temperature. Acclimation and recovery are not characterized by a complete return to the control state as low molecular weight heat shock mRNAs continue to associate with the polyribosomal and non-polyribosomal fractions.;Heat shock and control mRNAs are differentially distributed in the non-polyribosomal RNP of control cells. However, messages active in translation at 25{dollar}\sp\circ{dollar}C are primarily associated with cytoskeletally-associated polyribosomes. Heat shock results in a change in both the distribution of heat shock mRNAs on the ribosomes of polyribosomes and their non-polyribosomal to polyribosomal ratios. Heat shock mRNAs accumulate primarily in the free-cytoplasmic non-polyribosomal and polyribosomal fractions suggesting that translation and possibly transport of these messages during heat shock is largely independent of the cytoskeleton. The negligible changes in the equilibrium of membrane-derived non-polyribosomal fractions together with the accumulation of mRNAs destined for the endoplasmic reticulum in free-cytoplasmic non-polyribosomal RNP are consistent with this interpretation. Neither the distribution of 25{dollar}\sp\circ{dollar}C mRNAs on polyribosomes nor their equilibrium between non-polyribosomal and polyribosomal RNP's is affected by the temperature shift.;The kinetics of association/dissociation of mRNAs with the non-polyribosomal and polyribosomal RNP indicate that heat shock messages associate with non-polyribosomal RNP prior to their integration into polyribosomes.



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