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Journal of Virology, January 2000, p. 146-155, Vol. 74, No. 1
Laboratory of Infectious Diseases, National
Institute of Allergy and Infectious Diseases, National Institutes
of Health, Bethesda, Maryland 20892-0720,1 and
Department of Immunology/Microbiology,
Rush-Presbyterian-St. Luke's Medical Center, Chicago, Illinois
606122
Received 22 June 1999/Accepted 20 September 1999
The 3' termini of the genomic and antigenomic RNAs of
human respiratory syncytial virus (RSV) are identical at 10 of the
first 11 nucleotide positions and 21 of the first 26 positions. These conserved 3'-terminal sequences are thought to contain the
genomic and antigenomic promoters. Furthermore, the
complement of each conserved sequence (i.e., the 5' end of the RNA it
encodes) might contain an encapsidation signal. Using an RSV
minigenome system, we individually mutated each of the last
seven nucleotides in the 5' trailer region of the genome. We analyzed
effects of these mutations on encapsidation of the T7
polymerase-transcribed negative-sense genome, its ability to
function as a template for RSV-driven synthesis of positive-sense
antigenome and mRNA, and the ability of this antigenome to
be encapsidated and to function as template for the synthesis of more
genome. As a technical complication, mutations in the last five
nucleotides of the trailer region were found to affect the efficiency
of the adjoining T7 promoter over more than a 10-fold range, even
though three nonviral G residues had been included between the core
promoter and the trailer to maximize the efficiency of promoter
activity. This was controlled in all experiments by monitoring the
levels of total and encapsidated genome. The efficiency of
encapsidation of the T7 polymerase-transcribed genome was not affected
by any of the trailer mutations. Furthermore, neither the efficiency of
positive-sense RNA synthesis from the genome nor the efficiency of
encapsidation of the encoded antigenome was affected by the
mutations. However, nucleotide substitution at positions 2, 3, 6, or 7 relative to the 5' end of the trailer blocked the production of progeny
genome, whereas substitution at positions 1 and 5 allowed a low level
of genome production and substitutions at position 4 were tolerated.
Position 4 is the only one of the seven positions examined that is not
conserved between the 3' ends of genomic and
antigenomic RNA. The mutations that blocked the synthesis
of progeny genome thus limited RNA replication to one step, namely, the
synthesis and encapsidation of antigenome. Restoration
of terminal complementarity for one of the trailer mutants by making a
compensatory mutation in the leader region did not restore synthesis of
genomic RNA, confirming that its loss was not due to reduced
terminal complementarity. Interestingly, this leader mutation appeared
to prevent antigenome synthesis with only a slight effect on
mRNA synthesis, apparently providing a dissociation between these two
synthetic activities. Genomes in which the terminal 24 or 325 nucleotides of the trailer have been deleted were competent for
encapsidation and the synthesis of mRNA and antigenomic
RNA, further confirming that terminal complementarity was not required
for these functions.
0022-538X/0/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Mutations in the 5' Trailer Region of a Respiratory Syncytial
Virus Minigenome Which Limit RNA Replication to One Step
*
Corresponding author. Mailing address: Laboratory of
Infectious Diseases, NIAID, NIH, 7 Ctr. Dr., MSC 0720, Bethesda, MD
20892-0720. Phone: (301) 496-3481. Fax: (301) 496-8312. E-mail:
pcollins{at}atlas.niaid.nih.gov.
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