Abstract— Treatment of ultraviolet‐inactivated tobacco mosaic virus ribonucleic acid (TMV–RNA) with extracts obtained from the local lesion host, Nicotiana tabacum var. Xanthi, n.c., and simultaneous illumination at 365 nm results in up to a four‐fold increase in infectivity over non‐illuminated controls. The active material in the extract appears to be associated with protein, based on its inactivation by boiling, precipitation with ammonium sulfate, and exclusion from Bio‐Rad P100 polyacrylamide. Partially purified DNA photoreactivating enzyme from yeast or pinto bean has no activity on ultraviolet‐irradiated TMV–RNA.
It had been observed by one of us (C. A. Knight, unpublished) that green, yellow, and white areas of variegated Xanthi tobacco plants (Nicotiana tabacum L. vat. Xanthi nc) yield essentially the same numbers of lesions when inoculated with tobacco mosaic virus (TMV) or its RNA. Since the action spectrum for photoreactivation of viral RNA inactivated by ultraviolet radiation of wave length 254 rim. does not resemble the action spectrum for photosynthesis I969), it seemed possible that photoreactivation (PR) can occur on leaves devoid of chlorophyll. We have tested this hypothesis on white leaves of a variegated mutant of Xanthi tobacco (Burk, Stewart & Dermen, 2964) and have observed a photorecovery of 3o to 35 %.Variegated Xanthi plants were grown from seeds kindly provided by Dr H. Dermen. The stems of variegated plants which had been grown to a height of 15 to 30 cm. were cut into 6 to 8 cm. pieces and grafted on to normal green stocks of Xanthi tobacco. Green leaves, variegated leaves, and all-white leaves issued from the grafts. Only the white leaves were used in the present experiments. When the white leaves were 8 to 24 cm. in length, they were randomly marked for assays with u.v.-inactivated TMV RNA for both dark survival and survival with photoreactivation (PR) (Bawden & Kleczkowski, I959). The white leaves were dusted with 4oo-mesh carborundum and the plants were placed in a dark room illumi nated only with non-photoreactivating red light (Hidalgo-Salvatierra & McLaren, I969). The left half-leaf of each PR leaf, as viewed with the stem up, was inoculated with a suspension of unirradiated TMV-RNA at about 1.8 #g./ml. in o. i M-phosphate buffer (pH 7) containing bentonite at ~ mg./ml. Each corresponding right half-leaf was inoculated with a suspension containing bentonite at I mg.fml, and TMV-RNA at about 28/zg./ml. which had been irradiated with stirring at 254 nm. for 200 sec. at 31 x ~o -9 einsteins/ml./min. (o'o24J/ml.) in a quartz cuvette of I cm. path length; the survival of infectious TMV RNA under these irradiation conditions is about so %. The dilutions at which the various irradiated and unirradiated samples of RNA were applied to plants were chosen so as to yield similar numbers of lesions on the opposite half-leaves of the test plants. Each PR leaf was washed free of its carborundum and each whole plant was placed under four GE type F 4o BLB 'black lights' with an output of 4 x 10 -4 j/cm.2/sec, at the minimum to a x t o -~ j/cm)/sec, at the maximum for the range of distances of leaves from the lights (Hidalgo-Salvatierra & McLaren, I969). After 45 rain. the plants were brought out and the leaves which were marked as dark controls were inoculated exactly as above; the plants were washed free of carborundum, placed in darkness overnight and next morning transferred to the glasshouse in daylight. The following day the leaves showed lesions which were counted; the chlorophyll content of the white leaves was only about 2 % of that in normal green leaves.The results of three experiments are summa...
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