Different evolutionary behavior of structurally related, repetitive sequences occurring in the same Balbiani ring gene in Chironomus tentans.

The Balbiani ring 2 (BR 2) gene in Chironomus tentans is highly internally repeated. Two types of related repeat units-the alpha and beta types-are tandemly arranged in separate blocks, which together are likely to form the major part of the gene. Every repeat unit has one constant region and one subrepeat region. Here we analyze the length and sequence of a number of repeat units of both types and compare the units within and between the blocks. The approximately 100 alpha repeat units are essentially invariant regarding length and sequence. In contrast, when the approximately 70 beta repeat units are compared, six length variants are found, four of which have been sequenced. The length variations reside in the subrepeat regions and are due to different numbers of whole or half subrepeats. Furthermore, the subrepeat regions differ by several base-pair substitutions, many of which change the amino acid sequence. On the other hand, all beta-type constant regions are of equal length and are virtually homogeneous in sequence. The observed length distributions in combination with analysis of the basepair substitutions in the alpha-and beta-type constant and subrepeat regions suggest that the alpha and beta blocks are of different age, that seemingly homologous repeated regions may play different functional roles at the protein level, and that sequence correction mechanisms are likely to operate to different extents on the constant and subrepeat regions within the beta block.