Format: ORAL

Yurij O. Tynkevich, Irina I. Panchuk, Roman A. Volkov

Yuriy Fedkovych Chernivtsi National University, Chernivtsi, Ukraine

Tandemly arranged repetitive regions (repeats) that encode 5S rRNA (5S rDNA) are an obligatory component of eukaryotic genomes. Typically, the 5S rDNA repeats within a genome are very similar due to the concerted nature of their evolution. Each 5S rDNA repeat consists of a conserved coding sequence (CDS) and a rapidly evolving intergenic spacer (IGS). The high evolutionary stability of the CDS is the result of purifying selection to maintain the function of the 5S rRNA as a component of the ribosome. In contrast, it is believed that the main part of the IGS probably has no function and therefore evolves at a high rate. To uncover the patterns of IGS evolution in a broad taxonomic context, we applied cloning, sequencing, and bioinformatic analysis to describe the molecular organization of 5S rDNA in 31 genera representing 15 families and 12 orders of angiosperms. It was found that the short-sequence motifs at the 3 and 5 ends of the IGS, which are involved in the initiation and termination of 5S rDNA transcription, evolve at a relatively low rate and demonstrate apparent conservation within a genus/family. Surprisingly, our analysis also revealed that in eight genera studied the middle part of IGS harbors conserved sequence elements (CSE), 50 to 200 bp in length, which show clear similarity (70 to 91%) to the genomic sequences of taxonomically distant families. This may be due to presumptive horizontal gene transfer. Alternatively, the CSE may represent ancient sequences that were conserved in some phylogenetic lineages. The function of CSE and the mechanism of its maintenance during long-term evolution remain enigmatic and require further research.