Conserved and nonconserved structures in the secretory proteins encoded in the Balbiani ring genes of Chironomus tentans.

The large, repetitive Balbiani ring (BR) genes, BR 1, 2, and 6, in Chironomus tentans originated from a short ancestral sequence and have all evolved according to analogous amplification schemes. We analyzed the structures of the BR-encoded secretory proteins and defined the parts that have been conserved during the evolutionary process. The BR products show striking similarities, with the BR 1 and BR 2 products being more similar to each other than to the BR 6 product. In the constant (C) region of the repeat units, 7 of the 30 amino acid residues are strictly conserved; 4 of these are the cysteine residues. The subrepeat (SR) regions of all the BR products are dominated by repeated tripeptide elements rich in proline and charged amino acid residues. Most of the amino acid replacements in both regions are conservative. Secondary structure predictions suggested that the C regions of the BR 1 and BR 2 products have several elements of secondary structure: an alpha-helix, a beta-strand, and one or two reverse turns, as in "globular structures." The prediction for the C region of the BR 6 product is similar but lacks a beta-strand. The predictions for the intervening SR regions appear less conclusive, but are clearly different from those for the C regions, and suggest regular structures not differing in their conformational elements. The SR regions evolved from an ancestor sequence similar to the C region; thus, the BR products seem to represent an example of evolution from one structure to two differently folded products.(ABSTRACT TRUNCATED AT 250 WORDS)