The C-terminal DNA-binding domain of Chironomus BR gene products shows preferential affinity for (dA.dT)-rich sequences.

Balbiani ring genes (BRs), the most active loci in the polytene chromosomes of the salivary gland of the midge Chironomus (Diptera), code for secretory giant peptides (the sp-I family). Evidence previously reported indicated that the conserved C-terminal region of proteins of the sp-I family had DNA-binding properties (assayed with sp-Ia), and one such region, derived from BR2.2, which codes for the product sp-Ib, might occur as a stable independent peptide, being transferred to the nucleus where it is detectable in the large BRs (BR1 and BR2), among other structures, by immunostaining. Here, we show that the C-terminal portion of one of the BR gene products, expressed as a glutathione-S-transferase fusion protein shows preferential affinity for A.T-rich sequences and binds with varying affinity to restriction fragments of the A.T-rich BR1 promoter. The binding was inhibited by distamycin, suggesting that the interaction involves the minor groove of the DNA. Analysis of the promoter fragments by gel electrophoresis indicated that most appeared to present a conspicuous bend, as deduced from their anomalous electrophoretic mobilities. Furthermore, the affinity of the C-terminal domain for the different promoter fragments appeared to correlate with the degree of bending. Thus, the C-terminal domain might play a role in controlling gene expression by binding to A.T-rich sequences, including those of the BR genes.