Support plasmids and support proteins required for recovery of recombinantrespiratory syncytial virus

Respiratory syncytial virus (RSV) can be recovered from plasmids that separ ately encode antigenomic RNA and the N, P, L, and M2-1 proteins of the nucl eocapsid. However, in a recent study the inclusion of a separate M2-1 expre ssion plasmid was found to be unnecessary (H. Jin, D. Clarke, H. Zhou, X. C heng, K. Coelingh, M. Bryant, and S. Li, Virology 1998, 251, 206-214). This suggested that the M2-1 protein, which is a transcription antitermination factor, is not required to reconstitute the minimum unit of infectivity, na mely a nucleocapsid fully functional for viral transcription and RNA replic ation. Here we show that the antigenomic plasmid is remarkably efficient as a substitute for an M2-1 expression plasmid in supporting processive trans cription by an RSV minigenome. Thus, the simple expedient of omitting an ex pression plasmid is invalid for evaluating recovery requirements. The issue of the requirement of M2-1 for the recovery of infectious RSV is discussed .