There are five families of double-stranded RNA virus-like particles (L-A, L-BC, M, T, and W) and two distinct single- stranded RNA virus-like entities (208 RNA and 238 RNA) that replicate in cells of Saccharomyces cerevisiae. We have studied how these genomes replicate in yeast with emphasis on the role of the host. Highly purified virus-like particles (VLPs) carry out both (+) strand and (-) strand synthesis of L-A, L-BC, or M RNA in vitro in a conservative, sequential reaction. We can open L-A double-stranded RNA (dsRNA) VLPs by dialysis. They release their dsRNA, but now can use exogenous (+) strand of L-A or M as a template to make the corresponding dsRNAs in vitro by synthesis of (-) strands. Analysis by Western blots reveals a 180,000 dalton VLP protein that specifically binds L-A or M (+) single-stranded RNA. We have isolated, cloned, and sequenced a deletion mutant of the 4.5 kb L-A, called X, that is 530 bp long. X dsRNA is in VLPs with an L-A encoded coat and is transcribed and replicated in these VLPs. Thus the cis signals for these processes are in the X sequence. X has the same ends as the parent L-A molecule and lacks most of the center sequences. Unlike L-A, X is incompatible with Ml and requires many chromosomal genes that Ml, but not L-A, needs for its replication. Like Ml, X represses the L-A copy number. We suggest that molecules encoding the coat protein (L-A parent) need fewer MAK genes to protect them from SKI products than of molecules borrowing their cost protein from L-A (like Ml and X). We find that (B), a cytoplasmic gene suppressing M's requirement for many MAK genes, is located on certain L-A matural variants. Sequence data and gene fusion studies inidicate that the MAK11 product is a membrane-associated protien. The MAK16 gene is involved in the yeast cell division cycle and is necessary for passage through the "start" point at which cells are arrested by the mating pheromones. Our sequence of the CDC16 gene, which is involved in chromosome segregation, shows that the protein has three apparent zinc-binding--nucleic acid-binding "fingers."