A COLD-INDUCIBLE BOMBYX GENE ENCODING A PROTEIN SIMILAR TO MAMMALIAN SORBITOL DEHYDROGENASE - YOLK NUCLEI-DEPENDENT GENE-EXPRESSION IN DIAPAUSE EGGS

To facilitate the study of the induction of sorbitol dehydrogenase by acclimation to 5-degrees-C in diapause eggs of the silkworm, Bombyx mori, two cDNA libraries from eggs and larval fat bodies were screened with anti-(sorbitol dehydrogenase) serum, and a positive cDNA was cloned from the fat-body cDNA library. 1039 nucleotides determined from the cDNA corresponded to a protein-coding region consisting of 346 amino acids. The missing regions (containing two amino acids at the 5' end and a stop codon at the 3' end) were supplemented with the genome sequence. The deduced amino-acid sequence had 45 - 47 % identity with mammalian sorbitol dehydrogenases. The results led us to conclude that the cDNA for a Bombyx homolog of mammlian sorbitol dehydrogenase was isolated, which was designated as BmSDH. Analyses of Northern hybridization and reverse transcription/polymerase chain reaction showed that the transcript of BmSDH occurred after chilling for 40-50 days when the diapause eggs were exposed to 5-degrees-C from two days after oviposition to break the diapause. The changing pattern in the amount of BmSDH transcript was well correlated with those in the activity of sorbitol dehydrogenase and the amount of the enzyme protein in diapause eggs. Further, the transcript of BmSDH was localized in yolk cells. The results indicate that the yolk nuclei-dependent gene expression of BmSDH is induced by acclimation to 5-degrees-C in diapause eggs.