Control of the levels of alanyl-, glycyl-, and seryl-tRNA synthetases in the silkgland of Bombyx mori.

We have examined the levels of glycyl-, alanyl-, and seryl-tRNA synthetases and the levels of their corresponding translatable mRNAs in the posterior and middle silkglands of the silkworm, Bombyx mori. Analysis of Western blots reveals that the change in the abundance of these enzymes during the fifth instar in crude extracts derived from posterior and middle silkgland correlates with changes in enzymatic activity; most of the change in activity for seryl- and alanyl-tRNA synthetases can be accounted for by the corresponding change in enzyme concentration, while the apparent specific activity of glycyl-tRNA synthetase appears to be elevated in the posterior silkgland. Accompanying the changes in enzyme activity and enzyme concentration are changes in the levels of the corresponding mRNAs as determined by immunoprecipitation of in vitro translation products. The levels of all three enzymes are 10 to 20 times higher in the posterior and middle silkglands than in ovarian tissue. A form of alanyl-tRNA synthetase with a slightly higher apparent molecular weight is detected in the posterior silkgland early in the fifth instar and in ovarian tissue.