Relationship between small antisense RNAs and aberrant RNAs associated with sense transgene mediated gene silencing in tomato.

We investigated the occurrence and properties of small sense and antisense RNAs and aberrant transcripts in transgenic tomato plants exhibiting post-transcriptional silencing of the endogenous polygalacturonase (PG) sense gene and a truncated homologous transgene. Small antisense RNAs, about 23 nucleotides in size, were present in the leaves and immature green fruit where the transgene was transcribed in the absence of transcripts from the fruit ripening-specific endogenous PG gene. This suggests that the small antisense RNAs were produced from the transgene, which was supported by their failure to hybridise with the 3' half of the endogenous PG gene, the region that was excluded from the transgene. The features of two aberrant RNAs, corresponding to the 5' and 3' halves of the endogenous gene, strongly suggest that they arise by endonucleolytic cleavage of endogenous PG mRNA. The region where cleavage occurs is associated with the preferential production of small RNAs from the 3' end of the transgene. These results suggest a model where small antisense RNAs generated from the 3' end of the transgene anneal to the endogenous mRNA, resulting in cleavage at or near the annealing regions.