Abstract
The salivary gland cells of Chironomus larvae synthesize almost exclusively secretory proteins, for constructing the tubes in which they live. The secretory proteins of C. thummi contain a high percentage of basic amino acids, chiefly lysine (about 20%), and consist of two main giant secretory proteins of about 1,000 kd, called sp-Ia and sp-Ib, and a number of low-molecular-weight fractions, sp-180, sp-150, sp-36, sp-18, and sp-15. Galactose treatment shows that the synthesis of sp-I fractions is correlated with the puffing of Balbiani rings, BRb and BRc, and with the relative abundance of Balbiani ring RNA transcripts in the cytoplasm. The sp-I fractions, which appear to represent real polypeptides, display individual- and strain-specific size variations. Cleavage of sp-I proteins with cyanogen bromide (CNBr) yields peptides of about 17 kd and peptide ladders composed of 8–10 multimers of this 17kd “basic sequence”. These properties are typical for proteins encoded by the giant, internally repeated Balbiani ring genes.
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