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Abstract |
ABSTRACT: BACKGROUND: In plants, the 5S rRNA genes usually occur as separate tandems or, less commonly, linked to 35S rDNA units. The activity of linked genes remains unknown so far. We studied expression of 5S genes in several species from family Asteraceae known to contain linked 35S-5S units. Additionally, their methylation status was determined using bisulfite sequencing. Fluorescence in situ hybridization was applied to reveal sub-nuclear positions of rDNA arrays. RESULTS: We found that linked genes dominate 5S rDNA expression in species with prevalent 35S-5S rDNA arrangement. Several species evolved functional variants of the polymerase III promoter in which a residing C-box element differs from the canonical angiosperm motif by as much as 30%. On this basis, a more relaxed consensus sequence of a plant C-box: (5'-RGSWTGGGTG-3') is proposed. The 5S paralogs display heavy DNA methylation similarly as their unlinked counterparts. FISH revealed close association of 35S-5S arrays with nucleolar periphery indicating that transcription of 5S genes may occur in this subnuclear territory. CONCLUSIONS: We showed that the unusual linked arrangement of 5S genes, occurring in several plant species, is fully compatible with their expression and functionality. The extraordinary 5S genes dynamics is manifested at different levels, such as changes in chromosomal positions, unit structure, epigenetic modification and considerable divergence of regulatory motifs. |
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