POTATO SPINDLE TUBER VIRUS - A PLANT VIRUS WITH PROPERTIES OF A FREE NUCLEIC ACID .1. ASSAY EXTRACTION AND CONCENTRATION

Relative virus concentration in extracts prepared from tissue infected with potato spindle tuber virus (PSTV) was estimated by use of an infectivity index which is based on the dilution of inoculum, the time required for symptom expression on tomato plants, and the number of plants infected. Expressed sap or extracts prepared with buffers of low ionic strength (0.005 M K2HPO4) contained little infectivity (dilution end point 10-1); extracts prepared with buffers of high ionic strength (0.5 M K2HPO4) were highly infectious (dilution end points 10-3 to 10-4). Most infectious material remained in the supernatant after high-speed centrifugation (1-2 hours at 40,000 rpm) and in the upper and middle fractions of sucrose density gradients that were centrifuged for 16 hours at 24,000 rpm. Infectious material could be concentrated by precipitation with ethanol (dilution end points up to 10-7). Treatment with phenol had no appreciable adverse effect on the infectivity of PSTV preparations. © 1969.