For the maintenance of "killer" M1 double-stranded RNA in Saccharomyces cerevisiae, more than 30 chromosomal genes are required. The requirement for some of these genes can be completely suppressed by a cytoplasmic element, [B] (for bypass). We have isolated a mutant unable to maintain [B] (mab) and found that it is allelic to MAK10, one of the three chromosomal MAK genes required for the maintenance of L-A. The heat curing of [B] always coincided with the loss of L-A. To confirm that [B] is located on L-A, we purified viral particles containing either L-A or M1 from strains with or without [B] activity and transfected these purified particles into a strain which did not have either L-A or M1. The transfectants harboring L-A and M1 from a [B] strain showed the [B] phenotype, but the transfectants with L-A and M1 from a [B-o] strain did not show the [B] phenotype. Furthermore, the transfectants having L-A from a [B] strain and M1 from a [B-o] strain also showed the [B] phenotype. Therefore, we concluded that [B] is a property of a variant of L-A. In the transfection experiment, we also proved that the superkiller phenotype of the [B] strain is a property of L-A and that L-A with [B] activity can maintain a higher copy number of M1 regardless of the source of M1 viruslike particles. These data suggest that MAK genes whose mutations are suppressed by [B] are concerned with the protection of M1 (+) single-stranded RNA or the formation of M1 viruslike particles and that an L-A with more efficient production of M1 viruslike particles can completely dispense with the requirement for those MAK genes.
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