Structure of the smallest salivary-gland secretory protein gene in Chironomus tentans

The salivary gland secretion in the dipteran Chironomus tentans is composed of approximately 15 different secretory proteins. The most well known of the corresponding genes are the four closely related Balbiani ring (BR) genes, in which the main part of each approximately 40-kb gene is composed of tandemly arranged repetitive units. Six of the seven additional secretory protein genes described share structural similarities with the BR genes and are members of the same BR multigene family. Here we report the identification of a new secretory protein gene, the sp12 gene, encoding the smallest component of the C. tentans salivary gland secretion. The gene has a corresponding mRNA length of approximately 0.7 kb and codes for a protein with a calculated molecular weight of 7,619 Da. The sp12 gene was characterized in seven Chironomus species. Based on a comparison of the orthologous gene sequences, we conclude that the sp12 gene has a repetitive structure consisting of diverged 21-bp-long repeats. The repeat structure and the codon composition are similar to the so-called SR regions of the BR genes and the sp12 gene may represent a diverged member of the BR multigene family.