A REPETITIVE SECRETORY PROTEIN GENE OF A NOVEL TYPE IN CHIRONOMUS-TENTANS IS SPECIFICALLY EXPRESSED IN THE SALIVARY-GLANDS AND EXHIBITS EXTENSIVE LENGTH POLYMORPHISM

The secretion from the salivary glands in the dipteran Chironomus tent ans has previously been shown to contain secretory proteins encoded by a set of related genes belonging to the Balbiani ring multigene famil y. Here we describe the characterization of a gene for an additional s ecretory protein of a novel type. This sp240/420 gene is built from vi rtually identical 477-base pair repeats organized in tandem. The gene exhibits extensive length polymorphism since allelic length variants w ith from 12 to 22 repeats were recorded. Antibodies have been raised a gainst the gene product, and it is shown also that the protein varies in size between 240 and 420 kDa. The protein is rich in serine and thr eonine residues (30%) and has N-linked carbohydrate chains, presumably on each repeat. Despite careful preparation of the gland lumen protei ns, the sp240/420 protein was found to form protein ladders of regular ly degraded protein, suggesting that this degradation occurs naturally in the salivary glands. Although all previously characterized salivar y gland secretory protein genes in C. tentans belong to the same multi gene family, the sp240/420 gene appears to represent a novel, unrelate d type of repetitive gene. The chromosomal location of the gene was ma pped and was found to reside in region 17 on chromosome I.