Light microscope autoradiography of ribosomal DNA amplification in Xenopus laevis

A technique for the isolation of very high molecular weight rDNA¹ from the ovary of Xenopus laevis is described. Tritiated rDNA was prepared by this method from ovaries at the amplification stage, and spread on slides for light microscope autoradiography. The average molecular weight of the spread DNA was greater than 180×10⁶ daltons. Unlike chromosomal DNA grain tracks, rDNA tracks after 2 or 4 hours of labelling were not tandemly arranged. By allowing ovaries to equilibrate gradually with exogenous precursors, tracks showing a single gradient of grain density were produced, indicating that replication was proceeding in one direction at these sites. Bidirectional initiations, if they occur at all during amplification, are rare. The rate of rDNA chain growth is 10.5 μ/hour at 23° C, which is the same as the rate for chromosomal DNA synthesis in X. laevis. After 24 hours some tracks are over 200 μ long, suggesting that replication at a site may be continuous for at least this period. Although they do not distinguish between several alternative mechanisms, the results are compatible with a rolling circle mechanism for gene amplification.     

The genes for ribosomal RNA in diploid and polytene chromosomes of Drosophila melanogaster

The proportion of the Drosophila genome coding for ribosomal RNA was examined in DNA from both diploid and polytene tissues of Drosophila melanogaster by rRNA-DNA hybridization. Measurements were made on larvae with one, two, three and four nucleolus organizer regions per genome. In DNA from diploid tissues the percent rDNA (coding for 28S and 18S ribosomal DNA) was found to be in proportion to the number of nucleolus organizers present. The number of rRNA genes within a nucleolus organizer therefore does not vary in response to changes in the number of nucleolus organizers. On the other hand, in DNA from cells with polytene chromosomes the percent rDNA remained at a level of about 0.1% (two to six times lower than the diploid values), regardless of either the number of nucleolus organizers per genome or whether the nucleolus organizers were carried by the X or Y chromosomes. This independence of polytene rDNA content from the number of nucleolus organizers is presumably due to the autonomous polytenization of this region of the chromosome. When the rDNA content of DNA from whole flies is examined, both the rDNA additivity of the diploid cells and the rDNA independence of polytene cells will affect the results. This is a possible explanation for the relative rDNA increase known to occur in X0 flies, but probably not for the phenomenon of rDNA magnification. — In further studies on DNA from larval diploid tissues, the following findings were made: 1) the Ybb-chromosome carries no rDNA; 2) flies carrying four nucleolus organizers do not tend to lose rDNA, even after eleven generations, and 3) the nucleolus organizer on the wild type Y chromosome may have significantly less rDNA than does that on the corresponding X chromosome.     

Localization of rDNA in the chimpanzee (Pan troglodytes) chromosome complement

In situ hybridization was used to identify the sites of rDNA in the chromosome complement of the chimpanzee (Pan troglodytes). The rDNA was present in the satellite regions of chimpanzee chromosomes 14, 15, 17, 22 and 23. Four of these (14, 15, 22, 23) are homologous to human chromosomes carrying rDNA: 13, 14, 21 and 22.     

The chromosomal location of ribosomal DNA in the mouse

In situ hybridization of I¹²⁵-labelled ribosomal RNA to mouse chromosomes was used to determine the location of the rDNA loci. The results demonstrate the presence of rDNA sites on chromosomes 15, 18 and 19.     

Isolation and characterisation of immunoglobulins in the serum of precolostral piglets

The presence of actively synthetized immunoglobulin in the serum of newborn precolostral germfree piglets was confirmed. This immunoglobulin, being of IgG antigenic type, carries determinants of typical IgG heavy and light chains and has a sedimentation constant of 4S. The first antibodies formed in germfree piglets after immunization with sheep red blood cells are of macroglobulinemic (IgM) nature. They are followed by formation of more slowly sedimenting antibodies of the IgG type. No fast sedimenting antibodies of the IgG type were detected.     

Translation of hepatic mRNA in extracts from wheat germ embryos

Extracts from wheat embryos have been used to study the incorporation of amino acids into TCA insoluble products using hepatic mRNA fractions. The properties of this system are described and compared to the incorporation obtained with poly U and rabbit globin mRNA. SDS-acrylamide gel analysis showed that the major polypeptide synthesized with globin mRNA co-migrates with rabbit globin (15 500 daltons). Rat liver products were numerous, with molecular weights from less than 10000 to greater than 65000 daltons. The KCl concentration for maximum incorporation into TCA precipitable polypeptides with hepatic mRNA was not the optimum KCl concentration for synthesis of complete products.     

Biological activity of 125iodine labelled globin mRNA in an Ehrlich ascites cell-free system

Globin mRNA was isolated from avian erythroblasts and labelled with ¹²⁵iodine to a specific activity of 4.5×10⁶ dpm μg^-1 mRNA. The labelling procedure was modified as compared to common techniques in order to protect the mRNA against degradation. When this RNA is translated in an Ehrlich ascites cell-free system globin chains are synthesized as is demonstrated by the analysis of the in vitro synthesized products. The biological activity of ¹²⁵iodine labelled mRNA has proven to be 30% of that of unlabelled mRNA.     

Immune response in human subjects at high altitude

Levels of immunoglobulins were determined in persons exposed to high altitude. The individuals studied included high altitude natives, sea level residents at high altitude for 2 years, and recent arrivals at high altitude. Increased IgG and IgA levels were found in high altitude natives and sea level residents at high altitude for 2 years when compared with sea level residents. In recent arrivals marked increase of IgG and IgG levels and slight rise in IgM was seen. Recent arrivals who suffered from high altitude pulmonary oedema showed marked elevation of IgG, IgM, and IgA. Immunoglobulin responses to both primary and secondary TAB inoculation were of a higher magnitude and more sustained at high altitude than at sea level.