Location of rRNA genes in three inbred strains of rat and suppression of rat rRNA activity in rat-human somatic cell hybrids

Nucleolus organizer regions (NORs) of rat chromosomes were stained by the Ag-AS method. The Ag-NORs were found on chromosomes 3, 11 and 12 in the ACI, Wistar Brown and Wistar Lewis inbred strains of rat. The size of the Ag-NOR on each pair of chromosomes varied from strain to strain. Rat-human somatic hybrid cells that retained human and lost some of the rat chromosomes had no Ag-NOR on rat chromosomes 3, 11 or 12. Since NORs can be Ag-stained only if their 18 + 28S rRNA genes are active, the activity of the rat rRNA genes must have been suppressed in the hybrid cells.     

Segregation of rat chromosomes in somatic cell hybrids between rat cells and HT 1080 human fibrosarcoma cells

Summary We produced somatic cell hybrids between HT 1080-6TG human fibrosarcoma cells and either rat white blood cells (WBC) or cells directly derived from rat spleen. Karyologic and isozyme analyses of hybrid cells indicated that they preferentially lose rat chromosomes. Hypoxanthine-aminopterine thymidine-selected hybrid clones expressing rat hypoxanthine phosphoribosyltransferase (HPRT), glucose-6-phosphate dehydrogenase (G6PD), and phosphoglycerate kinase (PGK) and containing the rat X chromosome were counterselected in a medium containing 30 g/ml of 6-thioguanine. Concordant loss of the rat X chromosome and of the expression of rat HPRT and G6PD was observed in the hybrid clones.     

Assignment of the gene for cytoplasmic glutamic-oxaloacetic transaminase to the region q-24-qter of human chromosome 10.

Somatic cell hybrids between thymidine kinase-deficient mouse cells and human fibroblasts carrying a translocation of the distal third of the long arm of chromosome 10 to chromosome 17 were studied for the expression of cytoplasmic glutamic-oxaloacetic transaminase. A positive correlation between the expression of human cytoplasmic glutamic-oxaloacetic transaminase and the presence of the distal third of the long arm of chromosome 10 was established.     

The major antenna complex of photosystem II has a xanthophyll binding site not involved in light harvesting.

We have characterized a xanthophyll binding site, called V1, in the major light harvesting complex of photosystem II, distinct from the three tightly binding sites previously described as L1, L2, and N1. Xanthophyll binding to the V1 site can be preserved upon solubilization of the chloroplast membranes with the mild detergent dodecyl-alpha-d-maltoside, while an IEF purification step completely removes the ligand. Surprisingly, spectroscopic analysis showed that when bound in this site, xanthophylls are unable to transfer absorbed light energy to chlorophyll a. Pigments bound to sites L1, L2, and N1, in contrast, readily transfer energy to chlorophyll a. This result suggests that this binding site is not directly involved in light harvesting function. When violaxanthin, which in normal conditions is the main carotenoid in this site, is depleted by the de-epoxidation in strong light, the site binds other xanthophyll species, including newly synthesized zeaxanthin, which does not induce detectable changes in the properties of the complex. It is proposed that this xanthophyll binding site represents a reservoir of readily available violaxanthin for the operation of the xanthophyll cycle in excess light conditions.