NOVEL VARIANTS OF THE 5S RIBOSOMAL-RNA GENES IN ERUCA-SATIVA

The 5S ribosomal RNA (rRNA) genes of Eruca sativa were cloned and char acterized. They are organized into clusters of tandemly repeated units . Each repeat unit consists of a 119-bp coding region followed by a no ncoding spacer region that separates it from the coding region of the next repeat unit. Our study reports novel gene variants of the 5S rRNA genes in plants. Two families of the 5S rDNA, the 0.5-kb size family and the 1-kb size family, coexist in the E. sativa genome. The 0.5-kb size family consists of the 5S rRNA genes (S4) that have coding region s similar to those of other reported plant 5S rDNA sequences, whereas the 1-kb size family consists of the 5S rRNA gene variants (S1) that e xist as 1-kb BamHI tandem repeats. Si is made up of two variant units (V1 and V2) of 5S rDNA where the BamHI site between the two units is m utated. Sequence heterogeneity among S4, V1, and V2 units exists throu ghout the sequence and is not limited to the noncoding spacer region o nly. The coding regions of V1 and V2 show approximately 20% dissimilar ity to the coding regions of S4 and other reported plant 5S rDNA seque nces. Such a large variation in the coding regions of the 5S rDNA unit s within the same plant species has been observed for the first time. Restriction site variation is observed between the two size classes of 5S rDNA in E. sativa. The noncoding spacers of the variants V1 and V2 that make up the 1-kb family lack the EcoRI site that is present in t he 0.5-kb family. The sequence analysis indicates that V1 and V2 seque nces are probably pseudogenes derived from functional 5S rRNA genes. T he results also suggest that the two families exist as independent clu sters at different locations in the E. sativa genome.