From the cDNAs of two defective RNAs naturally exhibiting a large difference in replication efficiency, a series of Sendai virus RNA chimeras were constructed by reciprocal exchanges of their 3' end primary sequences. Using a reverse genetics system, the ability of these RNAs to replicate when expressed from cDNAs in the context of the viral proteins N, P, and L, also expressed from plasmids, was analyzed. First the extent of potential RNA 3'/5' end complementarity was tested by disrupting and restoring the terminal 110-nucleotide complementarity of a copy-back RNA. Alternatively, this base pairing potential was gradually increased from 12 to 57 or to 98 nucleotides by continuous substitutions. In all cases, the restoration or the creation of more extended base pairing potential had no effect on RNA replication. Reciprocal exchanges were then made in order to identify cis-acting sequences that could induce high replication efficiency. It was found that nucleotides 1-31 of the antigenome 3' end were sufficient to confer a high replication property (more than a 10-fold increase), regardless of the sequence adjacent to these terminal nucleotides. It is concluded that one of the most important features that modulate replication efficiency is contained in the promoter end primary sequence and that this feature is likely to operate independently of the ability to form a potential terminal base pairing.