Mario Buongiorno-Nardelli

Size of ribosomal DNA repeating units in Xenopus laevis: Limited individual heterogeneity and extensive population polymorphism

Purified somatic ribosomal DNAs of 45 Xenopus laevis adults have been separately analyzed by agarose gel electrophoresis after Eco RI digestion. A pattern of gene length heterogeneity, unique for each individual, has been observed. This length heterogeneity, known to involve the spacer, is limited to few different size classes (up to about 10). The analysis of ribosomal DNA purified from ovaries indicates that, during oogenesis, amplification involves preferentially, if not exclusively, only some size classes of genes. An electron microscope study of reannealed molecules of individual high molecular weight ribosomal DNA demonstrates a tandem organization of equally sized genes, forming homogeneous blocks within the nucleolar organizer.

Isolation and structural analysis of ribosomal protein genes in Xenopus laevis: Homology between sequences present in the gene and in several different messenger RNAs☆

Abstract The ribosomal protein genes are present in two to four copies per haploid genome of Xenopus laevis. Using cloned complementary DNA probes, we have isolated, from a genomic library of X. laevis, several clones containing genes for two different ribosomal proteins (L1 and L14). These genes contain intervening sequences. In the case of the L1 gene, the exons are 100 to 200 base-pairs long and the introns, on average, 400 base-pairs. Along the genomic fragments, two different classes of repetitive DNA are present: highly and middle repetitive DNA. Both are evolutionarily unstable as shown by hybridization to Xenopus tropicalis DNA. Several introns of the gene coding for protein L1 contain middle repetitive sequences. Hybridization and hybrid-released translation experiments have shown that sequences inside the two genes hybridize to several poly(A) messenger RNAs. Some of the products encoded by these mRNA have electrophoretic properties of ribosomal proteins.

An electron microscope study of chromosomal DNA replication in different eukaryotic systems

Abstract We have carried out an electron microscope study of replication intermediates in chromosomal DNA from V. faba root tips; D. melanogaster, X. laevis and chick at different developmental stages; goldfish and brown trout germinal and somatic tissues; regenerating rat liver; Chinese hamster and X. laevis cultured cells. Clusters of small eyes (microbubbles, 100–300 base pairs) and single-stranded DNA molecules have been observed in all the systems analysed, whereas long replication eyes (macrobubbles) were present only in early cleavage embryos of D. melanogaster . The relative frequency of these structures has been compared with similar data from the literature and their relevance as replication intermediates discussed.

Molecular hybridization of Chinese hamster 5S, 4S and "pulse-labelled" RNA in cytological preparations.

Abstract 5S and 4S RNA gene localization was studied by the in situ molecular hybridization technique in hamster liver and brain sections. 5S RNA genes appear to be localized mainly in the nucleolar or perinucleolar region. 4S RNA genes seem to have a similar localization although the higher background level makes the evaluation of the localization more difficult. On the other hand, hybridized “pulse-labelled” RNA appears, as expected, not to be preferentially confined in any particular nuclear structure.

Relationship between the Organization of DNA Loop Domains and of Replicons in the Eukaryotic Genome

The DNA in the eukaryotic nucleus has been found to be arranged in a series of supercoiled loops. This supercoiled loop-organization results from DNA attachment to a structure which is mainly proteic, the so-called nuclear matrix (1–9). This DNA organization remains stable throughout the cellular cycle as far as the size of DNA loops and the anchorage sites are concerned (5, 10, 11). It is worthwhile noting that this type of organization has been found in all cells whatever their state of differentiation (early embryonic stage up to fully differentiated cells (21)).

Electron microscopic analysis of DNA replication in eukaryotes

Abstract An electron microscopic analysis of replication intermediates in chromosomal DNA from several systems has been made. In all of them, besides entirely double stranded molecules, only molecules with clustered small “eyes” and partially or entirely single stranded molecules were observed. Early cleavage embryos of D. melanogaster alone showed the presence also of molecules with long replication “eyes”. These patterns strongly support a unifying interpretation we recently proposed for chromosomal DNA replication in eukaryotes. As for the replication of specific chromosomal genes, a more refined approach has led us to envisage the rRNA gene cluster in X. laevis as a multi-replicon complex, the origins of replication being confined to the non-transcribed spacers.