A 28-fold increase in secretory protein synthesis is associated with DNA puff activity in the salivary gland of Bradysia hygida (Diptera, Sciaridae)
AbstractWhen the first group of DNA puffs is active in the salivary gland regions S1 and S3 of Bradysia hygida larvae, there is a large increase in the production and secretion of new salivary proteins demonstrable by [ 3 H]-Leu incorporation. The present study shows that protein separation by SDS-PAGE and detection by fluorography demonstrated that these polypeptides range in molecular mass from about 23 to 100 kDa. Furthermore, these proteins were synthesized mainly in the S1 and S3 salivary gland regions where the DNA puffs C7, C5, C4 and B10 are conspicuous, while in the S2 region protein synthesis was very low. Others have shown that the extent of amplification for DNA sequences that code for mRNA in the DNA puffs C4 and B10 was about 22 and 10 times, respectively. The present data for this group of DNA puffs are consistent with the proposition that gene amplification is necessary to provide some cells with additional gene copies for the production of massive amounts of proteins within a short period of time (Spradling AC and Mahowald AP (1980) Proceedings of the National Academy of Sciences, USA, 77: 1096-1100).
A differential inhibition of the synthesis of secretory proteins, mainly fractions formed in giant translation units, can be obtained in Chironomus salivary gland cells with low concentrations of the ribosome translocation inhibitor, cycloheximide with or without emetine. Both treatments also lead to puff regression and inhibition of transcription specific to the large Balbiani rings, BR1 and BR2, the loci for the giant secretory proteins. The amount of 75S BR RNA transcribed is also reduced in the cytoplasm and in the poly(A) RNA relative to other transcripts. The half-life of 75S RNA is, however, prolonged so that there is little if any decrease in the cytoplasmic content of 75S RNA. The effect on the Balbiani rings may be due to control emanating from the translational process.
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