Organization of ribosomal genes in Paramecium tetraurelia.

Open Access
Publisher Website
Google Scholar
Abstract
The macronuclear rDNA of the ciliated protozoan P. tetraurelia (stock 51) was analyzed by digestion with restriction endonucleases. The fragments which contained rRNA coding sequences and spacer sequences were identified. The spacer sequences exhibited some heterogeneity in size. The genes coding for 5.8S RNA, but not for 5S RNA, are linked to the 17S and 25S rRNA genes. Complementary RNA, synthesized from rDNA of stock 51, was hybridized with restriction digests of whole cell DNA from 6 other allopatric stocks of this species. The restriction patterns of the rDNA from these 7 stocks were, in general, very similar, and the sizes of the coding sequences were identical in all 7 stocks. Only the restriction pattern of rDNA from stock 127 differed significantly from that of stock 51. The rDNA from stock 127 was isolated and characterized, and with the exception of the restriction pattern of its spacer, it resembled the rDNA from stock 51. The rDNA repeat in Paramecium, including the spacer, has, in general, been conserved during the course of evolution. In some species, even in the absence of genetic exchange among geographically separated populations, selection pressure may act to conserve spacers of tandemly repeated rDNA. The conservation may be related to the number of rDNA copies in the germinal nucleus.