Constant and variable parts in the Balbiani ring 2 repeat unit and the translation termination region

Abstract

The large Balbiani rings of the chironomids produce giant internally repeated transcripts that are translated into silk‐like proteins used for protective tubes. We have cloned fragments of the Balbiani ring 2 (BR2) gene of Chironomus pallidivittatus, normally the most prominent BR, for sequencing and restriction analysis. The results indicate a basic, tandemly arranged repeat unit of 198 bp, consisting of an invariant region of 102 bp followed by a variable region of 96 bp, the latter containing short internal tandem repeats. In the coding strand of both regions, there is a tendency to maximize adenine (˜40%) and minimize cytosine + thymidine (32‐33%). Stringency in codon usage and absence of preference for third position nucleotide substitutions between homologous sequences suggest selection at the nucleotide level to be important. Both regions code for peptides rich in basic amino acids, but show distinct differences in other coding properties. The invariant region probably codes for a crystalline domain, and the variable region for a proline‐rich, amorphous domain. One of the clones includes the 3′ end of the translated region. Surrounding and following two stop codons is a sequence of four short palindromes. Furthermore, the first stop codon is part of a sequence reminiscent of a “TATA box”. The possibilities are discussed that this area of the gene might be a target for regulatory molecules controlling translation termination and/or the expression of an overlapping cistron.

Balbiani ring induction in phosphate metabolism

“…By assuming a size of 37 kb for BR1 as for BR2 in C. tentans (28), the cloned repeat represents about 40% of the gene. Only one open reading frame was present with a constant and a subrepeated part as in other BRs (11,27,(29)(30)(31)(32).…”

“…It represents a repeat unit of 195 bp. It is member of a family of repeats among which the constant parts show close to 100% homology and the subrepeated parts show only about 80% homology (11). To (25) and hybridized with 32P-labeled plasmid DNA containing BR gene fragments (26): for BR1, pCp9O; for BR2, pCp31 (11); and for BR6, pCpl6-7.…”

“…To (25) and hybridized with 32P-labeled plasmid DNA containing BR gene fragments (26): for BR1, pCp9O; for BR2, pCp31 (11); and for BR6, pCpl6-7. Lanes: 3, 4, and 8, complete digests of total DNA with Hinfl; 2 and 7, partial digests (0.5x) obtained by shorter digestion times [the partial digestion for the BR2 gene has been shown (11) pCp9o AAA CCA GAG AAA TGC GGT AGC GCA ATG AGA CGT GTT GAA GGA GAG AGA TGT GCA…”

“…Total larval DNA was cut with EcoRI restriction endonuclease and separated on linear 5-24% NaCl gradient (11). Fractions enriched for BR sequences were identified by agarose gel electrophoresis and Southern blotting (75S [32P]RNA as probe).…”

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

“…The BR genes contain the genetic information for large salivary secretory proteins (8,9), which are used to spin tubes housing the aquatic larvae. The principal structures of the BR 1 (10)(11)(12) and BR 2 (13-15) genes in C. tentans, the BR 2 gene in Chironomus pallidivittattus (16), and the BR b (17) and BR c (18) genes in Chironomus thummi have been revealed, and, in all cases, the genes consist of a large number of 150-to 300-base-pair (bp)-long repeat units that are tandemly repeated. All repeat units have one constant region (C region) and one subrepeat region (SR region),t the latter being formed by a number of subrepeats.…”