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Potato spindle tuber viroid (PSTVd) is one of the smallest (about 360 nt) infectious plant agents. It is composed of a single-stranded circular non-coding RNA molecule. In the course of previous passage experiments with two intermediate PSTVd variants I2 and I4, three non-infectious clones (I2-50, I4-37 and I4 VI-17) were found. When inoculated separately as cDNAs on tomato “Rutgers” test plants these variants did not induce any visible disease symptoms and did not produce progeny. The presence of such non-infectious variants raises several questions about their origin and biology and to answer them, mixed co-infections with cDNA copies of two non-infectious variants (I2-50, I4-37) were performed. PSTVd infection was observed in seven out of 30 inoculated plants. The progeny isolated from three separate plants contained novel variants, together with the parental I2 and I4 sequences. It is conceivable that the appearance of repaired PSTVd molecules, clearly capable of cell-to-cell movement leading to the systemic infection, results from recombination events. An analysis of the recombinant molecules and comparison with databases identified the specific sites responsible for the restricted infectivity of the I2-50 and I4-37 PSTVd variants. In parallel experiments in which (+) strand PSTVd infectious transcripts were used, no recombinants were observed, and the original I2-50 and I4-37 non-infectious sequences were not detected in the progeny.
Yeast mitochondrial DNA codes for eight major polypeptides. Translation of he mitochondrially encoded polypeptides in strains with mutated mitochondrial release factor, mRF1, was found to result in the synthesis of a novel protein, V2. Different mrf1 alleles were associated with different efficiency of V2p synthesis. Translation of V2p was enhanced by paromomycin. Comparative analysis of peptides resulting from protease digestion indicated that V2p is a derivative of Var1p. According to our hypothesis, V2p represents a readthrough product of the natural stop codon in VAR1 mRNA.
The genetic con ti nu ity of the po tato spin dle tu ber viroid (PSTVd) ge nome was ana­lysed af ter in fec tion of to mato plants with cloned cDNAs of pa ren tal strains. Dur ing the six weeks of the experiment, several new sequence variants appeared. The se­quence vari ants de tected in the prog eny pop u la tion in duced se quence-specific dis ease symptoms. The PSTVd genome therefore follows the pattern expected for typical pseudo-strains prop a gat ing in plants as a pop u la tion of sim i lar se quences. As sessing fur ther the replicon con ti nu ity, a PSTVd cDNA mu tant with a de le tion in the cen tral con served re gion was con structed and proven to be non-infectious. Sur pris ingly, in a sub-population of po tato transformants ex press ing the same de leted PSTVd RNA an in fec tious viroid was detected. This suggests specific transcript conversion followed by recovery of the full-length pathogen genome.
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