5S рДНК тимофіївки лучної Phleum pratense L.

Abstract

Genomic regions encoding 5S ribosomal RNA (5S rDNA) are necessary for ribosomal biogenesis. Tandemly arranged repeated units of 5S rDNA found in the genomes of all eukaryotes represent a universal model for studying the evolution of repeated sequences. Each of the 5S rDNA repeated units consists of evolutionarily conserved sequences coding for ribosomal 5S rRNA and more rapidly evolving intergenic spacer regions (IGS). The sequence elements of the external promoter and terminator of RNA polymerase III, which are required for transcription of 5S rDNA, are located in the IGS. However, these motifs may vary in different families of angiosperms. The family Poaceae appears to be an attractive group of plants for the comparative study of 5S rDNA. However, for this family, the 5S rDNA still remains poorly studied. In particular, nothing is known about the molecular organization of 5S rDNA in such a widespread and economically important genus as Phleum. Accordingly, it was decided to clone, sequence, and analyze the structure of the 5S rDNA repeated unit of the timothy-grass Ph. pratense L. It was shown that only one variant of the 5S rDNA repeated unit with 120 bp-long coding region and 209 bp-long IGS is present in the genome of this species. The central part of the IGS consists of five tandemly arranged diverged subrepeats, which differ by nucleotide substitutions and small deletions/insertions. At the beginning of the IGS, an oligo-T sequence of the transcription terminator of RNA polymerase III is present. The conservative motives ATAA, GC and C, which represent the putative elements of the external 5S rDNA promoter, were detected in the IGS 28, 15 and 1 bp upstream of coding region. A comparative analysis of the obtained sequences shows that the location of the GC motive 15 bp upstream of the coding region is not found in the previously investigated species of angiosperms, i.e., it appears to be unique. The obtained results indicate the difference in the RNA polymerase III transcription initiation in dicots and monocots. Keywords: 5S rDNA, intergenic spacer, molecular evolution, repeated sequences, Phleum pratense, Poaceae