[Intragenomic polymorphism of the primary structure of 5S rRNA gene variants of the loach (Misgurnus fossilis L.). Determination of the transcriptional activity].
The primary structure of 12 cloned repeats of loach oocyte 5S rRNA genes was determined. The heterogeneity of nucleotide sequences was revealed in the coding regions and spacer of the genes. The results of the study on in vivo transcriptional activity of the cloned 5S rRNA gene variants are consistent with the localisation of site specific base substitutions in the coding part affecting the transcription. We have compared the nucleotide sequences of loach 5S rRNA gene variants and of Xenopus laevis, X. borealis and Bombyx mori 5S genes which can be actively transcribed in X. laevis oocyte nuclei. As a result we could propose a consensus nucleotide sequence in the internal control region (from 45-th up to 100-th nucleotide) of the eukaryotic 5S rRNA gene. This sequence comprises a RNA-polymerase III promotor and stretches interacting with transcriptional factors. We have considered the base substitutions in the nucleotide sequences of 5S gene variants exerted on the experimental model of loach 5S rRNA secondary structure. All base substitutions in actively transcribed genes do not influence the general double-stranded structures of the transcripts. However in 5S RNA transcripts from genes with low transcriptional activity base substitutions affecting the box c RNA-polymerase III promoter destroy hairpin II interacting with ribosomal proteins. We have concluded that two factors can restrict the divergency of 5S rRNA genes: (1) conservation of the nucleotide sequence in the gene internal control region, and (2) conservation of the general double stranded structures in 5S rRNA transcripts.