Differential regulation of small heat-shock genes in plants: analysis of a water-stress-inducible and developmentally activated sunflower promoter

We have isolated two sunflower genes, Ha hsp 18.6 G2 and Ha hsp 17.7 G4, that encode small heat shock proteins (sHSPs). RNAse A protection experiments, carried out with RNA probes transcribed from each gene and hybridized to sunflower total RNA, allowed us to distinguish their mRNA accumulation patterns. In sunflower, Ha hsp 17.7 G4 mRNAs accumulated during zygotic embryogenesis at 25°C. In vegetative tissues, these mRNAs accumulated in response to either heat shock (42°C), abscisic acid (ABA), or mild water stress treatments. In all cases, the mRNAs were transcribed from the same initiation site. In contrast, Ha hsp 18.6 G2 mRNAs accumulated only in response to heat-shock. This result demonstrates differential regulation of these two sHSP genes. The complex regulation depicted by the Ha hsp 17.7 G4 promoter has been further analyzed in transgenic tobacco, using G4::GUS translational fusions. Developmental induction of Ha hsp 17.7 G4 during zygotic embryogenesis was faithfully reproduced in the transgenic plants. 5-distal sequences (between -1132 and -395) were required to confer a preferential spatial expression of GUS activity in the cotyledons. More proximal sequences (from -83 to +163) conferred to the chimeric genes most of the developmental regulation, and the responses to ABA and heat shock characteristic of the Ha hsp 17.7 G4 promoter. The water stress response of this gene was not reproduced in transgenic tobacco and, thus, could be uncoupled from its regulation during embryogenesis.     

A heat shock-resistant mutant of Saccharomyces cerevisiae shows constitutive synthesis of two heat shock proteins and altered growth

A heat shock-resistant mutant of the budding yeast Saccharomyces cerevisiae was isolated at the mutation frequency of 10(-7) from a culture treated with ethyl methane sulfonate. Cells of the mutant are approximately 1,000-fold more resistant to lethal heat shock than those of the parental strain. Tetrad analysis indicates that phenotypes revealed by this mutant segregated together in the ratio 2+:2- from heterozygotes constructed with the wild-type strain of the opposite mating type, and are, therefore, attributed to a single nuclear mutation. The mutated gene in the mutant was herein designated hsr1 (heat shock response). The hsr1 allele is recessive to the HSR1+ allele of the wild-type strain. Exponentially growing cells of hsr1 mutant were found to constitutively synthesize six proteins that are not synthesized or are synthesized at reduced rates in HSR1+ cells unless appropriately induced. These proteins include one hsp/G0-protein (hsp48A), one hsp (hsp48B), and two G0-proteins (p73, p56). Heterozygous diploid (hsr1/HSR1+) cells do not synthesize the proteins constitutively induced in hsr1 cells, which suggests that the product of the HSR1 gene might negatively regulate the synthesis of these proteins. The hsr1 mutation also led to altered growth of the mutant cells. The mutation elongated the duration of G1 period in the cell cycle and affected both growth arrest by sulfur starvation and growth recovery from it. We discuss the problem of which protein(s) among those constitutively expressed in growing cells of the hsr1 mutant is responsible for heat shock resistance and alterations in the growth control.