Small infectious RNAs, such as satellites and viroids, can interfere with fundamental but as yet unidentified molecular processes in plant cells. Much attention has been focused recently on the generation of mature forms of circular small infectious RNAs. The discovery that some satellites and viroids undergo self-cleavage reactions while others only process in the presence of nuclear extracts is increasing our basic understanding of the multifunctional properties of RNA molecules. To study the generation of linear satellite RNAs, the turnip crinkle virus satellite system is ideal because of the multiple related satellites, the infectivity of in vitro altered satellite and the recent identification of a region essential for normal monomeric satellite accumulation. By in vitro alteration of the satellite sequence, followed by in vitro transcription of infectious cRNA and inoculation in plants, the nucleotides involved in satellite processing in this region as well as others will be precisely identified. To better understand this potentially novel processing mechanism, sequences involved in processing will be duplicated, deleted and moved to different locations in the satellite molecule. To aid in data analysis, an in vitro processing system will be developed using extracts from infected and uninfected plants. The study of small pathogenic RNAs is a particularly challenging and intriguing area of plant biology. Even though these RNAs can be described in molecular terms, their mode of action remains unsolved. The fact that they can have significant effects on plant development suggests that these simple pathogenic RNAs must somehow interact and interfere with fundamental molecular mechanisms in the normal plant cell.
