The relationship between symptom development and changes in the fast and slow electrophoretic forms of the two cadang-cadang disease-associated RNAs (ccRNA-1 and ccRNA-2) has been examined. The fast form of each is present in trees for up to 2 years before the appearance of symptoms, indicating an incubation period of about 2 years. Trees showing the first symptoms (on the nuts) contain only the fast form of the ccRNAs; the development of leaf symptoms coincides with the first detection of the slow form. After this time, both slow and fast forms are found in the newly developing fronds for the next 9 to 12 months but then, as symptoms develop, new fronds contain only the slow form. Therefore, at later stages of disease, only the slow form can be detected in all fronds. The fast form of ccRNA-1 and/or ccRNA-2 therefore appears to be the infectious form in nature. The number of nucleotides in the four ccRNAs as determined by polyacrylamide gel electrophoresis under denaturing conditions are: ccRNA-1 fast, 250; ccRNA-1 slow, 300; ccRNA-2 fast, 500; ccRNA-2 slow, 600. On the basis of two-dimensional fingerprints of ribonuclease A and T digests of the four ccRNAs, it was concluded that the ccRNAs are composed of repeated sequences of ccRNA-1 fast, and each of the ccRNA-2 forms is a dimer of the respective ccRNA-1 form.


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