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Journal of Virology, December 2000, p. 11017-11026, Vol. 74, No. 23
Laboratory of Infectious Diseases, National
Institute of Allergy and Infectious Diseases, Bethesda, Maryland
20892-0720
Received 24 May 2000/Accepted 12 September 2000
The intergenic sequences (IGS) between the first nine genes of
human respiratory syncytial virus (RSV) vary in length from 1 to 56 nucleotides and lack apparent conserved sequence motifs. To investigate
their influence on sequential transcription and viral growth,
recombinant RSV strain A2, from which the SH gene had been deleted to
facilitate manipulation, was further modified to contain an M-G IGS of
16, 30, 44, 58, 65, 72, 86, 100, 120, 140, or 160 nucleotides. All of
the viruses were viable. For viruses with an M-G IGS of 100 nucleotides
or more, plaque size decreased with increasing IGS length. In this same
length range, increasing IGS length was associated with modest
attenuation during single-step, but not multistep, growth in HEp-2
cells. Surprisingly, Northern blot analysis of the accumulation of six
different mRNAs indicated that there was little or no change in
transcription with increasing IGS length. Thus, the RSV polymerase
apparently can readily cross IGS of various lengths, including
unnaturally long ones, with little or no effect on the efficiency of
termination and reinitiation. This finding supports the view that the
IGS do not have much effect on sequential transcription and provides
evidence from infectious virus that IGS length is not an important
regulatory feature. To evaluate replication in vivo, BALB/c mice were
infected intranasally with RSV containing an M-G IGS of 65, 140, or 160 nucleotides. Replication of the latter two viruses was decreased up to
5- and 25-fold in the upper and lower respiratory tracts, respectively, on day 3 following infection. However, the level of replication at both
sites on days 4 and 5 was very similar to that of the virus with an IGS
of 65 nucleotides. Thus, the modest attenuation in vivo associated with
the longer IGS was additive to that conferred by deletion of the SH
gene and might be useful to incrementally increase the level of
attenuation of a live-attenuated vaccine virus.
0022-538X/00/$04.00+0
Respiratory Syncytial Virus Can Tolerate an Intergenic Sequence
of at Least 160 Nucleotides with Little Effect on Transcription or
Replication In Vitro and In Vivo
*
Corresponding author. Mailing address: Building 7, Room
100, NIAID, NIH, 7 Center Dr. MSC 0720, Bethesda, MD 20892-0720. Phone: (301) 594-1590. Fax: (301) 496-8312. E-mail:
pcollins{at}niaid.nih.gov.
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