Larval salivary gland secretion proteins in Drosophila. Identification and characterization of the Sgs-5 structural gene.

The 90BC locus on the polytene chromosomal map of Drosophila melanogaster contains the structural gene for a third-instar, salivary gland-specific, polyadenylated RNA (the group V RNA). This also belongs to the intermolt puff set whose dispersed and co-ordinately regulated members are (1) transcriptionally active in the salivary gland during the third-instar developmental stage and (2) comprise (at least in part) the structural genes for a set of salivary gland secretion proteins. Previous developmental studies of the group V intermolt gene (located cytogenetically within the 90B3-8 interval) suggest that it controls the expression of a salivary gland secretion protein. By analyzing different D. melanogaster laboratory stocks for variation in group V gene expression, we have been able to correlate the presence of the group V RNA with the salivary gland secretion protein P4. In vitro translation experiments show that the salivary gland messenger RNA population derived from a stock that fails to synthesize the group V RNA does not direct the synthesis of a polypeptide similar in molecular weight to protein P4. In addition, cloned genomic DNA segments complementary to the group V RNA are capable of arresting the in vitro translation of this protein. Comparative two-dimensional fractionation of cysteine-labeled, protease-generated peptides shows that (1) the in vitro translation product arrested by group V gene DNA is biochemically very similar to or identical with the salivary gland secretion protein P4, and (2) protein P4 is equivalent to the salivary gland secretion protein previously designated SGS-5. Since designations of the latter type have been employed in naming the genetic loci that represent the structural genes for the salivary gland secretion protein gene set, the group V gene (previous designation) represents the SGS-5 structural gene and its appropriate genetic designation should now be Sgs-5.