The experimental conditions were the same as those reported. To and then the 5Ј-and 3Ј-half fragments were separated by 10% PAGE fish out the aminoacyl-tRNAs, we designed two 3Ј-biotinylated DNA containing 7 M urea (10ϫ10 cm). Four hundred and thirty micrograms probes: 5ЈAGCAAGCTCAATGGATTCTGCGTCC3Ј for C.cylindracea of the 5Ј-half and 520 µg of the 3Ј-half fragments were recovered from tRNA Ser CAG and 5ЈGAAGCCCAATGGAACCTGCATCC3Ј for the gel. The purified 5Ј-half fragment was dephosphorylated with C.zeylanoides tRNA Ser CAG. These probes were immobilized with strepbacterial alkaline phosphatase (Takara Shuzo), and G33 at the 3Ј-end of tavidin agarose (Gibco BRL) as reported previously (Wakita et al., 1994) . the 5Ј-half fragment was removed by oxidation with sodium periodate as described in the literature (Keith and Gilham, 1974) . After dephos- A universal BMV-based RNA recombination system—how to search for general rules in RNA recombination At present, there is no doubt that RNA recombination is one of the major factors responsible for the generation of new RNA viruses and retroviruses. Numerous experimental systems have been created to investigate this complex phenomenon. Consequently, specific RNA structural motifs mediating recombination have been identified in several viruses. Unfortunately, up till now a unified model of genetic RNA recombination has not been formulated, mainly due to difficulties with the direct comparison of data obtained for different RNA-based viruses. To solve this problem, we have attempted to construct a universal system in which the recombination activity of various RNA sequences could be tested. To this end, we have used brome mosaic virus, a model (+)RNA virus of plants, for which the structural requirements of RNA recombination are well defined. The effectiveness of the new homomolecular system has been proven in an experiment involving two RNA sequences derived from the hepatitis C virus genome. In addition, comparison of the data obtained with the homomolecular system with those generated earlier using the heteromolecular one has provided new evidence that the mechanisms of homologous and non-homologous recombination are different and depend on the virus' mode of replication. RNA recombination is a very common phenomenon. It has been observed in all types of viruses using RNA as a carrier of genetic information: in positive-sense, single-stranded RNA viruses (1) (2) (3) (4) , in negative-sense, single-stranded RNA viruses (5, 6) , in double-stranded RNA viruses (7, 8) and in retroviruses (9) (10) (11) . Moreover, it has been shown that RNA recombination enables the exchange of genetic material not only between the same or similar viruses but also between distinctly different viruses (12) . Sometimes it also permits crossovers between viral and host RNA (13) (14) (15) (16) (17) . Taking into account the structure of viral genomic molecules and the location of crossover sites, three basic types of RNA recombination were distinguished: homologous, aberrant homologous and non-homologous (3, 4, 18) . The former two occur between two identical or similar RNAs (or between molecules displaying local homology), while the latter involves two different molecules. Most of the collected data suggest that RNA recombinants are formed according to a copy choice model (4, 18) . A viral replication complex starts nascent RNA strand synthesis on one template, called RNA donor and then switches to another template, called RNA acceptor. Accordingly, two main factors are thought to affect RNA recombination: the structure of recombining molecules and the ability of the viral replicase to switch templates.