In Vivo synthesis of potato spindle tuber viroid: Kinetic relationship between the circular and linear forms

Abstract

Study of the de novo formation of circular and linear forms of potato spindle tuber viroid (PSTV) showed that linear PSTV arises in vivo from circular PSTV. PSTV was labeled with 32P i for different periods of time beginning on the first day of symptom expression in systemically infected tomato leaves. With short periods (2 hr) of 32P; incorporation, predominantly circular PSTV was labeled. With longer periods of time, the ratio of circular to linear PSTV gradually decreased and with labeling periods of 96 hr and longer, more than 50% of the PSTV-labeling was in the linear form. When 32P 1 was replaced by [[ 3H]uridine or any of the other three ribonucleosides, the formation of linear PSTV was also prevalent with very long periods of incorporation. PSTV was also labeled with 32P 1 in older plants, beginning 1 week after the first day of symptom expression. With 4 hr of incorporation, circular PSTV was predominantly labeled as it was on the first day of symptom expression. With increasing duration of incorporation, linear PSTV again gradually increased, and at 48 hr the amounts of circular and linear PSTV were equal. The nucleotide composition of 32P-labeled circular or linear PSTV formed in vivo for 12 hr was very similar or identical. These results suggest that circular and linear PSTV molecules are two forms of the same RNA and that linear PSTV molecules arise in vivo by cleavage of circular PSTV molecules.