Separation and infectivity of circular and linear forms of potato spindle tuber viroid

Potato spindle tuber viroid can be separated into two fractions by polyacrylamide gelelectrophoresis in the presence of formamide and urea. One fraction contains predominantly circular molecules; the second fraction contains almost exclusively linear molecules. The purity of the fractions was estimated by electron microscopy of formaldehyde-denatured molecules. The length distributions of denatured circular and linear molecules were determined. Both circular and linear molecules were found to be infectious.