Structural constraints in expansion segments from a midge 26S rDNA.

DNA sequences representing approximately 40% of the large-subunit rRNA gene from the lower dipteran Chironomus thummi were analyzed. Once aligned with their Drosophila counterparts, sequence and base content comparisons were carried out. Sequence identity was found to be high overall, except for six regions that displayed a local bias in nucleotide composition toward AT. These regions were identified as expansion segments D3, D4, D5, D6, D7a, and D12. Besides base sequence divergence, differences in length were observed between the respective variable domains of the two species, particularly for D7a. Prediction of secondary structure showed that the folding of the Chironomus expansion segments analyzed is in agreement with the general patterns proposed for eukaryotic LSU rRNA. The comparison with Drosophila revealed also that the Chironomus secondary structures of the variable domains are supported by multiple compensatory substitutions or even compensatory insertions. Chironomus D7a displayed an unusual structural feature with respect to the insect D7a models that have been inferred up to now. The structural constraint observed in the expansion segments of Diptera so distantly related as midges and Drosophila suggests that these regions contribute to some functional role. Concerning the D7a of insects so far analyzed, there can be, in addition to a conserved secondary structure, a nucleotide composition constraint that might be important for the process giving rise to the alpha and beta halves of the 26S rRNA.