Interference between coinoculated viroids.

Experiments were carried out to seek evidence of an interaction between two viroid RNAs introduced to tomato plants in the same inoculum. At the level of symptom expression, the severe isolate of potato spindle tuber viroid (PSTV) dominated the mild isolate. Seventy-five percent of the plants inoculated with a 100-fold excess of the mild isolate developed unattenuated symptoms of severe disease. Other experiments revealed that infectious RNA molecules transcribed from cloned DNA templates containing PSTV sequences reduced the level of hop stunt viroid (HSV) RNA present in nucleic acid extracts of plants which had been inoculated with a mixture of dimeric plus-strand transcripts of these two viroids. Plants inoculated with dual transcripts--containing two copies of PSTV linked to two copies of HSV--developed characteristic symptoms of severe PSTV. Dot hybridization demonstrated that only PSTV replicated to detectable levels in these plants. A likely interpretation of these results is that the HSV portion of the dual transcripts failed to replicate because of interference from PSTV. These results raise questions about how the process of viroid replication is related to symptom expression, and lead to suggested models for the effect of viroid-like RNAs in cells under both normal and pathogenic circumstances.

Intrachromosomal recombination in Saccharomyces cerevisiae: reciprocal exchange in an inverted repeat and associated gene conversion.

Intrachromosomal gene conversion has not shown a strong association with reciprocal exchanges. However, reciprocal exchanges do occur between intrachromosomal repeats. To understand the relationship between reciprocal exchange and gene conversion in repeated sequences the recombination behavior of an inverted repeat was studied. We have found that in one orientation a single copy of the kanr gene of the bacterial transposon Tn903 flanked by part of the inverted repeats IS903 does not give G418 resistance in Saccharomyces cerevisiae. A reciprocal exchange in the IS903 repeats inverts the kanr gene, which then gives G418 resistance in a single copy. Using this as a selection for intrachromosomal reciprocal exchange we have introduced multiple restriction site heterologies into the IS903 repeats and examined the crossover products for associated gene conversions. Approximately 50% of crossovers, both in mitosis and meiosis, were associated with a gene conversion. This suggests that these crossovers result from an intermediate that gives a gene conversion in 50% of the events, that is, both reciprocal exchange and gene conversion between repeated sequences have a common origin. The data are most consistent with a heteroduplex mismatch repair mechanism.