ABSTRACT Chironomus salivary glands contain a family of high M, (~1,000 x 103) secretion polypeptides thought to consist of three components: sp-la, sp-lb, and sp-lc. The use of a new extraction protocol revealed a novel high M, component, sp-ld. Results of a survey of individual salivary glands indicated that sp-ld was widespread in more than a dozen strains of C. tentans and C. pallidivittatus. Sp-ld was phosphorylated at Set residues, and a comparison of cyanogen bromide and tryptic peptide maps of 3zp-labeled polypeptides suggested that spla, sp-lb, and sp-ld are comprised of similar but nonidentical tandemly repeated amino acid sequences. We concluded that sp-ld is encoded by an mRNA whose size and nucleotide sequence organization are similar to Balbiani ring (BR) mRNAs that code for the other sp-I components. Furthermore, parallel repression of sp-lb and sp-ld synthesis by galactose led us to hypothesize that both of their genes exist within Balbiani ring 2. Unusual structural proteins are secreted by a pair of simple yet highly specialized salivary glands in aquatic larvae of the midge, Chironomus (for reviews, see references 17 and 18). The perimeter of each gland consists of 30-40 secretory cell
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