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J. Virol., 08 1996, 5075-5082, Vol 70, No. 8
SK Samal and PL Collins
Genome analogs ("minigenomes") of Sendai and measles viruses replicate
efficiently only if their nucleotide length is an even multiple of six, a
requirement called the rule of six (P. Calain and L. Roux, J. Virol.
67:4822-4830, 1993; M. S. Sidhu, J. Chan, K. Kaelin, P. Spielhofer, F.
Radecke, H. Schneider, M. Masurekar, P. C. Dowling, M. A. Billeter, and S.
A. Udem, Virology 208:800-807, 1995). The existence of a comparable
requirement was tested for respiratory syncytial virus (RSV), which also is
a member of family Paramyxoviridae and whose natural genome length also is
a multiple of six. An internally truncated analog of RSV positive-sense
replicative intermediate RNA (antigenome) bearing the chloramphenicol
acetyltransferase gene as a reporter was synthesized from cDNA in vitro.
This RNA was transfected into cells which were infected with RSV as a
helper. Miniantigenomic RNA was indistinguishable from previously studied
negative-sense minigenome RNA in its ability to participate in
transcription, RNA replication, and incorporation into transmissible
particles. Sixteen miniantigenomes which were of slightly different lengths
and which in aggregate represented multiples of a wide range of integers
including 1 to 15 were constructed. During transfection and two serial
passages, the various miniantigenomes were essentially indistinguishable
with regard to the efficiency of transcription, RNA replication, and
packaging into transmissible particles. Progeny minigenomes of six
different mutants were recovered postpassage, copied into cDNA, cloned, and
sequenced completely. The length of each of these RNAs was found to have
remained unchanged during replication and passage. Thus, RSV transcription
and replication appear to lack the requirement that the template length be
an even multiple of an integer such as six, which for Sendai and measles
viruses is obligatory for nucleocapsid function. Each of the in
vitro-synthesized miniantigenomes used in transfection contained a nonviral
extension of three nucleotides, GGG, on the 5' (leader) end contributed by
the T7 promoter. The termini of the recovered minigenomes were examined for
five mutants by RNA circularization followed by cDNA synthesis,
amplification, cloning, and sequencing. Unexpectedly, each recovered
minigenome contained the complement of this nonviral extension on the 3'
(leader) end, showing that it had been faithfully copied and maintained
during RNA replication and passage. The nonviral trinucleotide did not
appear to affect the activity of the template.
Copyright © 1996, American Society for Microbiology
RNA replication by a respiratory syncytial virus RNA analog does not obey the rule of six and retains a nonviral trinucleotide extension at the leader end
Regional College of Veterinary Medicine, University of Maryland, College Park 20742, USA.
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