Oocyte and somatic 5S ribosomal RNA and 5S RNA encoding genes in Xenopus tropicalis.

We have investigated the structure of oocyte and somatic 5S ribosomal RNA and of 5S RNA encoding genes in Xenopus tropicalis. The sequences of the two 5S RNA families differ in four positions, but only one of these substitutions, a C to U transition in position 79 within the internal control region of the corresponding 5S RNA encoding genes, is a distinguishing characteristic of all Xenopus somatic and oocyte 5S RNAs characterized to date, including those from Xenopus laevis and Xenopus borealis. 5S RNA genes in Xenopus tropicalis are organized in clusters of multiple repeats of a 264 base pair unit; the structural and functional organization of the Xenopus tropicalis oocyte 5S gene is similar to the somatic but distinct from the oocyte 5S DNA in Xenopus laevis and Xenopus borealis. A comparative sequence analysis reveals the presence of a strictly conserved pentamer motif AAAGT in the 5'-flanking region of Xenopus 5S genes which we demonstrate in a separate communication to serve as a binding signal for an upstream stimulatory factor.     

Some somatic sequences are absent or exceedingly rare in Xenopus oocyte RNA

A variety of Xenopus laevis cDNA clones derived from somatic cell RNAs were hybridized to oocyte pA⁺ RNA separated on Northern gels. We were unable to detect oocyte pA⁺ sequences complementary to three undefined tadpole cDNA clones. With one of these clones, a complex pattern of bands appears during embryogenesis. With the other two clones, a single band appears. Two additional tadpole clones hybridize to both oocyte and tadpole RNA, but yield a more complex RNA pattern from embryos than from oocytes. One of these additional tadpole clones has complementarity to actin DNA, suggesting that the additional RNA band which appears during embryogenesis is α-actin mRNA (E. A. Sturgess, J. E. M. Ballantine, H. R. Woodland, P. R. Mohun, C. D. Lane, and G. J. Dimitriadis, 1980, J. Embryol. Exp. Morphol.58, 303–320). We have also failed to detect hybridization to oocyte pA⁺ RNA with one vitellogenin and three adult globin cDNA clones. Reconstruction experiments with purified globin mRNA from anemic adult blood cells set the lower level of sensitivity for globin mRNA at one part in 10⁶. The data suggest that some Xenopus mRNA sequences are absent or very rare in the oocyte pA⁺ RNA population.     

Sequence variation and methylation of the flax 5S RNA genes.

The complete sequence of the flax 5S DNA repeat is presented. Length heterogeneity is the consequence of the presence or absence of a single direct repeat and the majority of single base changes are transition mutations. No sequence variation has been found in the coding sequence. The extent of methylation of cytosines has been measured at one location in the gene and one in the spacer. The relationship between the observed sequence heterogeneity and the level of methylation is discussed in the context of the operation of a correction mechanism.     

Two 5S genes are expressed in chicken somatic cells.

Two 5S RNA species were detected in chicken cells. 5S I RNA has the nucleotide sequence of chicken 5S RNA previously published by Brownlee et al. (1) and 5S II RNA differs from it by 10 mutations. The secondary structure of both species is compatible with that proposed for other eukaryotic 5S RNAs. 5S II RNA represents 50-60% of 5S I RNA. Both species were found in total chicken liver and brain and were present in polysomes in the same relative proportions. Only one 5S RNA species could be detected in rat liver and HeLa cells. Chicken is the first vertebrate described so far in which two 5S RNA genes are expressed in somatic cells.