Fragility of attenuated Rauscher leukemia virus

Some biochemical properties of virulent (RRL+) and attenuated (RCL-) Rauscher leukemia virus were compared. It was shown that the reverse transcriptase (RT) activity of 'aged' as well as 'fresh' RCL- virions was 25--30% of that found for RRL+. The thermal sensitivity of the RT was the same for both viruses. A 60--70S RNA could be extracted from 'aged' RRL+, while no high molecular weight RNA was obtained from 'aged' RCL-. After centrifuging in sucrose gradient, most of the RT activity and 3H-labeled RNA of 'aged' RRL+ was recovered at 1.14--1.16 g/cm3, while for 'aged' RCL- no labeled RNA, and at most 10% of the original RT activity were found in the same zone. The fragility seemed to increase in the course of aging, since 'fresh' RCL- banded at 1.14--1.16 g/cm3 as did 'fresh' RRL+. Also, 3H-labeled viral RNA was found in the viral bands, from which 60--70S RNA could be extracted. Molecular hybridizations showed that 20% of the nucleic acid sequences related to Rauscher leukemia virus found in the RNA of RRL+-infected cells were missing in the RNA of RCL--infected cells.