Episomic suppression of phenotype inSalmonella

Abstract

1. An auxotroph ofSalmonella typhimurium, pro-401, was isolated in a strain that was unstable at thesu-leuAlocus. The auxotrophy ofpro-401is probably due to the attachment of a controlling episome to the proline region of the genome where it suppresses gene expression.2. The controlling episome frequently transposes over short distances so that all clones consist of cells, mixed for the site at which the controlling episome is attached; homologous transductions yield prototrophs.3. The controlling episome can transpose to a different complementation group; homologous transductions yield abortive transductants; syntrophism occurs between cells that are ‘mutant’ in different complementation groups to give reversions consisting entirely of auxotrophic cells which are called auxotrophic reversions.4. The controlling episome transposes over very short distances and never to beyond the limits of this proline region of the genome; no wild-type reversions were found.5. The controlling episome can be located at relatively distant proline sites in different clones; prototrophs from transductions between clones that are separated by many subculturings can be 100 times more frequent than from homologous transductions.6. The controlling episome has its frequency of transposition to different complementation groups increased by UV; irradiation increases the frequency of auxotrophic reversions.7. The controlling episome continues to transpose in stored cells.8. The pattern of reversions ofpro-401is different in these studies from its pattern two years previously. This is discussed.