Herman Denis

Biochemical research on oogenesis: Oocytes of Xenopus laevis synthesize but do not accumulate 5S RNA of somatic type☆

Abstract The oocytes of amphibians and teleosts begin to accumulate 5S RNA several months before other components of the ribosomes become available. Two types of genes coding for 5S RNA are active during oogenesis of these animals. One type of genes is expressed only in oocytes. The other type is expressed in both oocytes and somatic cells. In this paper, we show that the oocytes of Xenopus laevis do not accumulate 5S RNA of somatic type. We conclude that the products of the two types of genes behave differently during oogenesis. One product is stored by the oocytes, whereas the other is not. The heterogeneity of 5S genes in Xenopus laevis might have arisen because oocytes and somatic cells needed different kinds of 5S RNA. These needs are met by molecules having different primary structures, different conformations, and different metabolic stabilities in vivo. We do not understand how these properties are related to one another.

Recherches biochimiques sur l'oogenèse: V — Comparaison entre le RNA 5 S somatique et le RNA 5 S des oocytes de Xenopus laevis

Summary 5 S RNA from oocytes of Xenopus laevis differs from somatic 5 S RNA by the degree of phosphorylation of its 5′-end. There is in somatic 5 S RNA a higher proportion of molecules carrying 2 or 3 phosphate groups on the 5′-terminal nucleotide. Somatic 5 S RNA was retarded by columns of Sephadex G-100 slightly less than oocyte 5 S RNA. 5 S RNA from somatic ribosomes was eluted from columns of methylated albumin-Kieselguhr far behind 5 S RNA from whole oocytes and from oocyte ribosomes. Ribosomes from early embryos were found to contain 2 types of 5 S RNA: [1] newly made 5 S RNA which is of somatic type and [2] bulk 5 S RNA which is of the oocyte type. Somatic 5 S RNA migrated in 7.4 p. cent polyacrylamide gels slightly ahead of oocyte 5 S RNA. This difference of electrophoretic mobility was suppressed neither by removal of the terminal phosphates by alkaline phosphatase nor by heat or urea denaturation. Electrophoresis in 12.3 p. cent gels revealed the existence of 2 distinct forms in somatic 5 S RNA. One of these had the same mobility as oocyte 5 S RNA. The other one migrated faster. Somatic and oocyte 5 S RNAs reacted differently to heat denaturation: after cooling both native forms of somatic 5 S RNA were replaced by a third one, which migrated faster, whereas most of the oocyte RNA remained in native form. When heated in saline, oocyte 5 S RNA showed a slightly higher hyperchromicity than somatic 5 S RNA. These results suggest that both the primary and the secondary structures are different in somatic and in oocyte 5 S RNAs.

Recherches biochimiques sur l'oogenèse: 4. Absence d'amplification des gènes organisateurs du RNA 5 S et du tRNA dans les petits oocytes de Xenopus laevis

Summary Transfer RNA and 5 S RNA from cultured cells of Xenopus laevis were found to hybridize to the same extent with somatic DNA as with DNA from ovaries of immature females. Under the same conditions 28 S RNA hybridized about 40 times as well with ovary DNA than with somatic DNA. 5 S and tRNA genes are therefore not amplified in previtellogenic oocytes of X. laevis. However, these oocytes accumulate far more 5 S RNA and tRNA than 28 S and 18 S RNA.

Evolution of the 5 S RNA genes in vertebrates

SummaryWe have built the phylogenetic tree of Vertebrate 5S RNA using the sequence data of thirteen species belonging to six groups. Evolution of the 5S genes has been very slow in Vertebrates since 90 residues are identical in all 5S RNAs which are presently sequenced.In Amphibians and Teleosts different 5S genes are active in oocytes and in somatic cells. This dual gene system has probably been acquired independently by Amphibians and Teleosts. In Amphibians, the oocyte-type 5S genes have evolved much faster than the somatic-type genes. This is not true in all species since the oocyte-type genes of one Teleost (Tinca tinca) have evolved more slowly than the somatic-type genes.There are in all Vertebrate 5S RNAs five complementary regions which can be base-paired. The sequence data are compatible with the three secondary-structure models that have been proposed for 5S RNA.