Cell-free synthesis of citrus exocortis viroid (CEV) in nuclei-rich preparations 
from infected Gynura aurantiaca was optimum at 18-24 degrees C. Incubation of 
reaction mixtures at higher temperatures (30-36 degrees C) resulted in an 
increase of CEV linear molecules and the recovery of incomplete or nicked newly 
synthesized RNA species. Although the Mg(2+) optimum (2.5-5 mM) for CEV 
synthesis was lower than that for total [(32)P]CMP incorporation (10 mM), the 
response to Mn(2+) ion was distinctly different. Whereas maximum total activity 
was observed in 1 mM Mn(2+) with a pronounced reduction (80%) in 5 mM Mn(2+), 
CEV synthesis was maintained in 1-15 mM Mn(2+). Inhibition of 
alpha-amanitin-sensitive CEV synthesis in 200 mM (NH(4))(2)SO(4) resembles the 
reaction of RNA polymerase II on a free nucleic acid template. However, 
detection of trace levels of alpha-amanitin-resistant CEV synthesis activity 
inhibited by low (NH(4))(2)SO(4) concentrations (25 mM) suggests the possible 
involvement of RNA polymerase I- and/or III-like activity.