Episomic suppression of phenotype inSalmonella

Abstract

Many of the apparent re-versions of the proline-requiring auxotroph of Salmonella typhimurlum, pro-401. are unable to grow in the absence of proline (auxotrophic re-versions), while in homologous transductions involving pro-401 both wild type and abortive transductants are recovered; the number of such wild type transductants is much greater when several generations of sub-culturing separate the donor and recipient cultures than when the same culture is used both as donor and recipient. It is proposed that the pro-401 auxotroph arose by the attachment of a transposable controlling episometo a wild type proline gene, thus suppressing the expression of that gene. When transposition to a different complementation group occurs, synthrophism results giving rise to an auxotrophic reversion, while in homologous transductions the wild type transductants arise by recombination occuring between two chromosomes having the controlling episome located at different sites, thus restoring the wild genotype.