Paradoxical effects of Sendai virus DI RNA size on survival: inefficient envelopment of small nucleocapsids

In the assembly of nonsegmented negative-stranded RNA viruses, such as Sendai virus, the envelopment process allows extensively deleted genomes to survive by transmission from cell to cell in virus particles. To assess the impact of the sizes of such defective-interfering (DI) genomes on their survival, we performed competition tests among various species. Among copy-back DI RNAs, a 450-base species was gradually eliminated from DI virions by a 1200-base species, and the latter was independently eliminated by a 2800-base species. In each case, the smaller RNA species was synthesized and encapsidated at least as efficiently as the larger species, revealing that the level of competition was at the envelopment step in virus assembly. In contrast to the results obtained with the copy-back DI RNAs, repeated high multiplicity passage of a family of four internally deleted RNAs eliminated all but the smallest species, comprising about 1600 bases. Both sets of findings can be reconciled by the hypothesis that the efficiency of DI nucleocapsid envelopment decreases progressively when the RNA is smaller than about 1600 bases.