Summary
An interspecies comparison was made between the 3′ ends of Balbiani ring genes fromChironomus. The comparison was focused on the BR2.2 gene, and a part at the 3′ end fromChironomus pallidivittatus (which included also a segment of the gene core) was cloned. Its sequence, and other previously published BR sequences from this species and fromChironomus tentans were used in the analysis.
The 3′ parts of these repetitive genes can be divided into a region belonging to the core of the genes followed by a terminal region. In the core region the repeats (each of which consists of a constant part and a subrepeated part) are highly similar and the constant parts show little interspecies differentiation. Furthermore, the two parts of the repeats are units in an evolutionary and probably also functional sense.
The terminal region contains modified constant units, usually isolated betwen acidic so-called cys regions, the whole arrangement lying upstream of an intron toward a 3′-terminal exon. Most of the modified constant units are mosaics in rates of evolution with stable outer quarters bordering to equally stable cys regions and a central half with a very high rate of evolution. One of the terminal units, present only in the BR2.2 gene and second from the end, differs distinctly not only from corresponding core units but also from other terminal units in the three normally active BR genes. It lies upstream of all cys regions and is evolutionarily conserved over most of its length.
Furthermore, two-dimensional protein structure prediction does not exclude an endoproteolytic cleavage site in this unit. Such a site appears unlikely in other terminal or core regions. This is of interest in view of evidence for intracellular cleavage of the BR2.2 terminal region with liberation of a part containing a DNA-binding domain (Botella et al. 1988).
All in all the fine anatomy of evolutionary changes at the BR gene termini shows interesting correlations with postulated functional relations and may have predictive value in the further functional analysis of this part of the gene.
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Silva, F.J., Botella, L.M. & Edström, JE. Functional analysis of the 3′-terminal part of the Balbiani ring 2.2 gene by interspecies sequence comparison. J Mol Evol 31, 221–227 (1990). https://doi.org/10.1007/BF02109499
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DOI: https://doi.org/10.1007/BF02109499