Identification of enhancer-like elements in the ribosomal intergenic spacer of Odontophrynus americanus 2n and 4n (Amphibia, Anura)

Repetitive elements are found in the ribosomal intergenic spacer (IGS) of m ost organisms. A particularly complex pattern of internal repetition occurs in the IGSs of O. americanus 2n and 4n, which are composed of several type s of BamHI subrepeats (B-SRs). The most repetitive one is approximately 87 bp long, and is highly represented in the IGS variants of these amphibians. Sequence analyses of six diploid and two tetraploid B-SRs show 87% and 86% homology, respectively, and related secondary structure predictions. The c omparison of the 2n and 4n B-SR sequences aligned with the 81 bp enhancer o f Xenopus laevis reveals 36% homology. Furthermore, other B-SR features lik e size, number, and secondary structures resemble those of Xenopus enhancer s, suggesting that B-SRs may function as regulators of O. americanus rDNA t ranscription. The present data also corroborate the close evolutionary rela tionship between 2n and 4n O. americanus species.