A NEW MEMBER OF THE BALBIANI RING MULTIGENE FAMILY IN THE DIPTERAN CHIRONOMUS-TENTANS CONSISTS OF A SINGLE-COPY VERSION OF A UNIT REPEATED IN OTHER GENE FAMILY MEMBERS

The known Balbiani ring (BR) multigene family members in the dipteran Chironomus tentans encode salivary gland secretory proteins in the siz e range between 38 and 1,000 kDa. The proteins interact to form protei n fibers used by the aquatic larvae to spin feeding and protective lar val tubes or pupation tubes. Here, we describe a new BR multigene fami ly member, the sp17 gene, which codes for an 89-amino-acid-long protei n with a relative mobility of 17k. The gene has a high content of char ged amino acid residues and consists of two structurally different hal ves. Five regularly spaced cysteine codons are present in the 5' half while the 3' half contains five proline codons. These two different ha lves exhibit similarities to the C and SR regions, respectively, which form the tandemly repeated units in the about 40-kb-long BR genes and which also, in different versions, are the building blocks of all gen es in the BR multigene family. In this multigene family, encoding inte racting structural proteins, the long BR genes with their 125-150 tand emly arranged repeat units as well as the short sp17 gene with its sin gle-copy version of such a repeat unit, have therefore evolved from a common ancestor.