A hierarchic arrangement of the repetitive sequences in the Balbiani ring 2 gene of Chironomus tentans.

One cloned cDNA sequence, pCt63, was used to characterize the repeated structure of the Balbiani ring 2 gene in Chironomus tentans. Although small in size (0.63 kb), the cDNA insert corresponds to a large portion (25 kb) of the BR2 gene (37 kb). Southern blotting experiments suggested that a large part of the BR2 gene consists of tandemly repeated units, each about 215 bp. Sequence analysis of the cDNA confirmed the repeated nature of the BR2 gene and revealed the internal structure of the repeat unit. Each such unit is composed of two regions of approximately equal length; one is highly ordered and built from about six 18 bp repeats, each consisting of a slightly diverged 9 bp duplication. The recorded hierarchic arrangement of the repetitive sequences in the BR2 gene and a specific pattern of base substitutions along the gene have enabled us to propose how a major part of the giant BR2 gene has evolved from a short primordial sequence, 110-120 bp in length.