Inhibition of alfalfa mosaic virus RNA and protein synthesis by actinomycin D and cycloheximide

Autonomously replicating RNA in mitochondria of maize plants with S-type cytoplasm

Mitochondria isolated from maize plants with S-type male-sterile cytoplasms are capable of synthesizing four species of RNA at concentrations of actinomycin D that eliminate all DNA-directed RNA synthesis. No RNA synthesis occurs under the same conditions with mitochondria from plants possessing normal (N) cytoplasm or with other subcellular fractions from plants with S cytoplasm. The actinomycin D-resistant RNA synthesis occurs within the mitochondria since the labeling of these species is unaffected by inclusion of RNase in the incubation medium and since they become completely sensitive to RNase upon lysis of the mitochondria with low concentrations of Triton X-100. Two of the actinomycin D-resistant products are double stranded. These are 2850 and 900 base pairs in length, whereas the remaining two are 2150 and 850 bases. The synthesis of all four RNAs occurs in at least five different accessions of S cytoplasm, suggesting it is a general feature of S mitochondria. The double-stranded RNAs show homology to single-stranded S mitochondrial RNA but not to N mitochondrial RNA. Our observations indicate that the replication of these RNAs occurs independently of mtDNA and that they thus represent a novel type of inheritable element in organelles, an RNA plasmid.

Nonstructural alfalfa mosaic virus RNA-coded proteins present in tobacco leaf tissue

The proteins synthesized under the direction of alfalfa mosaic virus RNAS in tobacco leaves have been examined under conditions of suppressed host protein synthesis. Besides the coat protein we could detect a 22K (K = apparent molecular weight in thousands), a 35K, and a set of 54K proteins. The 22K protein is serologically related to the coat protein. The 35K protein comigrated with the 35K protein whose synthesis is directed by RNA 3 in vitro The 54K proteins are serologically related to the 35K protein produced in vitro. Readthrough products of the 35K protein cistron into the coat protein cistron have been found previously in wheat germ extracts programmed with RNA 3. Two of these proteins comigrate with the 54K proteins synthesized in vivo. Since the 35K and the coat protein cistrons are read in different reading frames the formation of readthrough products is puzzling. In viruses with a tripartite genome the subgenomic mRNA for coat protein, RNA 4, is not known to be replicated as a separate genome entity. This might indicate that proteins synthesized by readthrough into the coat protein cistron play an essential role during replication, especially in the earliest phases.

Virus protein synthesis in alfalfa mosaic virus infected alfalfa protoplasts

Four proteins unique to virus infection were synthesized in alfalfa mosaic virus-infected alfalfa mesophyll protoplasts. These proteins, P1, P2, P3, and coat protein comigrated on electrophoresis with the major in vitro translation products of RNA 1, RNA 2, RNA 3, and RNA 4, respectively. P1, P3, and coat protein were observed at 5 hr post inoculation; P2 was detected at 9 hr post inoculation. The three nonstructural proteins accumulated most rapidly early in infection until about 15 hr post inoculation; stable protein levels were maintained thereafter. Coat protein accumulated rapidly until about 20 hr after inoculation. All four virus RNA species were detected in infected protoplasts by labelling with [3H]uridine. Ultraviolet irradiation of protoplasts prior to inoculation was necessary for virus protein detection, but it severely depressed the synthesis of RNA 1 and RNA 2 relative to RNA 3 and RNA 4.

Effect of actinomycin D on tobacco mosaic virus (TMV) accumulation in isolated tobacco protoplasts under varying light conditions

The effect on Tobacco mosaic virus (TMV) accumulation of actinomycin D (AMD) introduced shortly after inoculation into isolated tobacco protoplasts under varying light conditions was examined. The emission spectrum of the light source contained lines in the visible range and in the ultraviolet band (300-400 nm). AMD absorbed light in the visible (400-500 nm) and in the uv (200-400 nm) ranges. AMD substantially inhibited TMV multiplication in the light, and also when the protoplasts were incubated in petri dishes covered with a black filter that allowed only uv light in the range 260-390 nm to pass. In the dark, and also in petri dishes covered with blue or yellow filters that passed in the ranges 400-500 or 500-600 nm, respectively, AMD stimulated TMV reproduction. The suppression of TMV multiplication in isolated tobacco protoplasts was assumed to be associated with a photodynamic effect caused by absorption of uv light by AMD.

An alfalfa mosaic virus RNA 2 mutant, which does not induce a hypersensitive reaction in cowpea plants, is multiplied to a high concentration in cowpea protoplasts

A mutant of alfalfa mosaic virus (AMV), which in contrast to wild type (wt) can invade cowpea plants systemically, is replicated more efficiently in cowpea protoplasts than the wt. Mutant preparations isolated from infected cowpea protoplasts contained a higher amount of middle component (M, containing RNA 2) than wt preparations. Both in cowpea plants and in cowpea protoplasts a wt phenotype is obtained upon addition of wt M to this mutant, suggesting a correlation between the type of plant reaction evoked by the virus infection and the regulation of viral RNA synthesis.