J Virol, July 1998, p. 5408-5413, Vol. 72, No. 7
0022-538X/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Department of Genetics and Developmental Biology,1 Mary Babb Randolph Cancer Center,2 and Department of Microbiology and Immunology,3 West Virginia University, Morgantown, West Virginia 26506
Received 28 October 1997/Accepted 18 March 1998
It has been documented that spleen necrosis virus (SNV) can package
murine leukemia virus (MLV) RNA efficiently and propagate MLV vectors
to the same titers as it propagates SNV-based vectors. Although the SNV
packaging signal (E) and MLV packaging signal (
) have little
sequence homology, similar double-hairpin RNA structures were predicted
and supported by experimental evidence. To test whether SNV RNA can be
packaged by MLV proteins, we modified an SNV vector to be expressed in
an MLV-based murine helper cell line. Surprisingly, we found that MLV
proteins could not support the replication of SNV vectors. The decrease
in titer was approximately 2,000- to 20,000-fold in one round of
retroviral replication. RNA analysis revealed that SNV RNA was not
efficiently packaged by MLV proteins. RNA hybridization of the cellular
and viral RNAs indicated that SNV RNA was packaged at least 25-fold
less efficiently than MLV RNA, which was the sensitivity limit of the
hybridization assay. The contrast between the MLV and SNV packaging
specificity is striking. SNV proteins can recognize both SNV E and MLV
, but MLV can recognize only MLV
. This is the first
demonstration of two retroviruses with nonreciprocal packaging
specificities.
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