The 3' ends of two genes in the Balbiani ring c locus of Chironomus thummi.

The 3'-end sequences of two nonallelic genes derived from the Balbiani ring c (BRc) locus of Chironomus thummi are described. Only one of the genes appears to be transcribed abundantly in normal late larval salivary glands. The two sequences are highly similar, even in the 3' untranslated regions, but sharply diverge beyond the polyadenylation site. Together with evidence from the 3' ends of BR1 and BR2 genes of C. pallidivittatus and C. tentans, independently characterized by others, this result suggests the existence of a sequence-homogenization mechanism that operates across the 3' ends of all BR genes characterized to date. The 3'-terminal coding region of each BRc gene is divided into two portions by a short intron. The upstream portion is homologous to and continuous with the tandem repeats that make up the internal core of each BR gene; however, that portion is variant in sequence relative to the core, and apparently is not subject to the homogenization process that operates on the core repeats. The portion downstream of the intron encodes a unique, 111-residue polypeptide highly different from the rest of the BRc product. The evolution of the various segments of the BRc genes is discussed.