Nucleic acid and protein metabolism in yeasts, higher plants and plant virus infections
Fraser, R. S. S.
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Research on nucleic acid and protein metabolism in yeasts, higherplants and in viral infections of higher plants is described.The antibiotic lomofungin, and 8-hydroxyquinoline were studied asinhibitors of ENA synthesis in yeast, and their mode of action wasshown to be by chelation of bivalent cations required for ENA.polymerase activity. The turnover of messenger RNA in yeast wasstudied using 8-hydroxyquinoline, and a mechanism of translationalcontrol over yeast protein synthesis proposed.Research on rates of ribosomal and messenger RNA synthesis in fission yeast (Schizosaccharomyces pombe) and budding yeast(Saccharomyces cerevisiae) suggested that for both species, synthetic rates of both types of macromolecule doubled after the period of DNA synthesis in each cell cycle. A simple model relating DNA content(gene dosage), rate of transcription and observed patterns of enzyme accumulation during the cell cycle was proposed for S_. pombe. The model was tested and elaborated by study of mutant cells differing in their genetic control over cell size; by examining haploid and diploid cells, and by a mathematical simulation. The results suggested that a cell-size related control operated over the rate of transcription, and could be important in control of cell growth rate and balanced exponential growth.Experiments with the higher plant cell cycle used the synchronous cell divisions induced in explants of Jerusalem artichoke tuber byexcision and culture. Freshly cut explants contained polyadenylated messenger RNA and ribosomal RNA. Messenger RNA synthesis predominated in the first few hours of culture; later, ribosomal ENA was accumulated in a stepwise manner, but the periodicity was shown not to be cell-cycle related. Ribosomal RNA synthesis was not required for induction of cell division or for the net accumulation of protein that occurred during the first two divisions. Two periods of putative messenger ENA synthesis were identified which were required for the occurrence of the succeeding divisions.Tobacco mosaic virus ENA was shown not to be polyadenylated, and not to have detectable méthylation. A previous report of nucleoside residues with cytokinin activity in this ENA could not be confirmed.Tobacco mosaic virus infection of tobacco plants inhibited synthesis and decreased stability of chloroplast ribosomal ENAs. Synthesis of cytoplasmic ribosomal RNA was initially stimulated, but was inhibited during active virus multiplication. Cytoplasmic ribosomal RNA stability was increased by infection. Virus infection reduced the rate of host protein synthesis by 75%» hut did not alter the host polyadenylated messenger RNA content, suggesting a control at the translational level. Changes in concentration of abscisic acid were measured after infection, and shown to be important in control of growth and ENA metabolism in infected plants.Constitutive resistance to tobacco mosaic virus controlled by the Tm-1 gene in tomato, and induced resistance to the virus in tobacco were studied, and information gained on the mechanism involved in each resistance. Methyl benzimidazol-2yl carbamate was found to inhibit tobacco mosaic virus multiplication in tobacco.NA metabolism was studied in germinating S_. pombe ascospores,and in germinating tobacco and carrot seeds.Methods were developed for detection of polyadenylated messenger 3ENAs, and for radioassay of ahscisic acid in plant extracts.A review article discussed the involvement of plant growth regulators in control of plant-virus interactions.