Expression of the small heat shock genes during Drosophila development: comparison of the accumulation of hsp23 and hsp27 mRNAs and polypeptides.

Seven heat shock genes are clustered within 15 kilobases of DNA at the Drosophila melanogaster chromosomal site 67B. They show a complex pattern of expression in the absence of external stress during normal development of this organism. In this paper, we quantitatively compare the abundance of the messenger RNAs for these seven genes at all major stages of Drosophila development and then focus on hsp23 and hsp27 for which available antibodies allow the comparison between the accumulation of the mRNAs and that of their corresponding polypeptides. Transcripts for both genes are maximally abundant in white prepupae. We observe that the amount of hsp23 message decreases more rapidly than that of hsp27 mRNA throughout the pupal period. The maximal abundance of the proteins occurs at the middle of the pupal stage, when their corresponding RNAs have almost completely disappeared. The peaks of expression of the proteins are also broader than those of their transcripts, indicating that the half-lives of the polypeptides are longer. These observations suggest that complex mechanisms regulate the expression of the small heat shock genes during Drosophila development.