rs1 satellite phage promotes diversity of toxigenic vibrio cholerae by driving ctx prophage loss and elimination of lysogenic immunity in el tor biotype strains of toxigenic vibrio cholerae the ctx prophage often resides adjacent to a chromosomally integrated satellite phage genome rs1 which produces rs1 particles by using ctx prophageencoded morphogenesis proteins rs1 encodes rstc an antirepressor against the ctx repressor rstr which cooperates with the hostencoded lexa protein to maintain ctx lysogeny we found that superinfection of toxigenic el tor strains with rs1 followed by inoculation of the transductants into the adult rabbit intestine caused elimination of the resident ctx prophageproducing nontoxigenic derivatives at a high frequency further studies using reca deletion mutants and a cloned rstc gene showed that the excision event was reca dependent and that introduction of additional copies of the cloned rstc gene instead of infection with rs1 was sufficient to enhance ctx elimination our data suggest that once it is excised from the chromosome the elimination of ctx prophage from host cells is driven by the inability to reestablish ctx lysogeny while rstc is overexpressed however with eventual loss of the additional copies of rstc the nontoxigenic derivatives can act as precursors of new toxigenic strains by acquiring the ctx prophage either through reinfection with ctx or by chitininduced transformation these results provide new insights into the role of rs1 in v cholerae evolution and the emergence of highly pathogenic clones such as the variant strains associated with recent devastating epidemics of cholera in asia subsaharan africa and haiti