Heat-stress induced synthesis of chloroplast protein synthesis elongation factor (EF-Tu) in a heat-tolerant maize line

A heat-tolerant maize (Zea mays L.) line, ZPBL 1304, synthesizes a unique set of five heat-shock polypeptides of 45 kDa. Previous studies suggested that these polypeptides might play a role in the development of thermotolerance in maize (Ristic et al., 1996, J. Plant Physiol. 149:424-432; Ristic et al., 1998, J. Plant Physiol. 153:497-505). In the present study, we isolated these polypeptides, sequenced them, and investigated their subcellular distribution and origin. Of the five polypeptides of 45 kDa, three polypeptides, including the two most abundant ones, yielded amino acid sequences similar to the chloroplast and bacterial protein synthesis elongation factor (EF-Tu). This was further confirmed using an antibody raised against maize EF-Tu, which showed a very strong reaction with the 45-kDa heat-shock protein(s). Studies on subcellular distribution and origin revealed that the 45-kDa polypeptides were localized to the chloroplasts, and were likely of nuclear origin. A full-length maize EF-Tu cDNA (Zmeftul), previously isolated from the B73 line of maize, was used as a probe for northern blot analysis of RNA extracted from the ZPBL 1304 maize line (the nucleotide and deduced amino acid sequences of Zmeftul are 88% identical to the rice EF-Tu sequence). Northern blots showed a 1.85-fold increase in steady-state levels of EF-Tu mRNA during heat stress. An increase in EF-Tu transcript levels during heat stress was accompanied by increased levels of the EF-Tu protein. Isolated chloroplasts from heat-stressed plants also had higher levels of EF-Tu as compared to control chloroplasts. The maize EF-Tu polypeptides showed > 80% sequence similarity with the bacterial EF-Tu, which has recently been shown to function as a molecular chaperone and to play a role in the protection of other proteins from thermal denaturation (Caldas et al., 1998, J. Biol. Chem. 273:11478-11482). It is hypothesized that chloroplast EF-Tu of the ZPBL 1304 maize line plays an important role in the development of thermotolerance.