Trehalase is an important enzyme for understanding carbohydrate and energy metabolisms in insects since the enzyme hydrolyzes trehalose, as the hemolymph sugar of insects, and leads it to glycolysis. Recently, cloning and sequencing of the trehalase gene of the silkworm, Bombyx mori, were achieved, however, its expression mechanism is still unclear.

To elucidate which step (e.g. transcription, translation and post-translation) is the key for the expression, we determined the quantities of mRNA, protein and enzyme activity of Bombyx trehalase in several tissues taken out from 5^th instar larvae. Northern blot analysis showed that the mRNA was expressed in the midgut, Malpighian tubule and in the anterior silk gland, but not in the fat body, ovary, testis and in the nervous system. In addition, amounts of the mRNA increased 3 to 4-fold after the gut purge in the midgut and Malpighian tubule. Throughout the developmental process, the amount of the mRNA corresponded well to the amounts of the accumulated protein and to the enzyme activity, estimated by Western blot analysis and enzyme assay, respectively. These results predicted that the gene expression of Bombyx trehalase depends on the transcription quantitatively.

Moreover, in order to estimate difference in net amount of trehalase transcription between before and after the gut purge (day 7), we carried out run-on assay using nuclei isolated from the midgut on day 3 and day 8 of the 5^th instar. The net amount of the transcription on day 8 was 3 times higher than that on day 3. This is confirmed the results obtained from Northern blot analysis.

Consequently, we conclude that transcription is the most important step for the trehalase gene expression in the silkworm, Bombyx mori.