NONCOMPLEMENTING DIPLOIDS FROM BACILLUS-SUBTILIS PROTOPLAST FUSION - RELATIONSHIP BETWEEN MAINTENANCE OF CHROMOSOMAL INACTIVATION AND SEGREGATION CAPACITY

Fusions of Bacillus subtilis protoplasts from two genetically marked s trains produce noncomplementing heterodiploid bacteria. These noncompl ementing diploids (Ncds) early both parental chromosomes, but only one is expressed. Fusion products of strains polymorphic for NotI restric tion sites provide new physical evidence to support the conclusion tha t Ncds are not an artifact of cross feeding or cell adhesion. We show that reversible chromosomal inactivation can only account for the bipa rental trait of unstable Ncds. Two types of cells were recovered from the late progeny of unstable Ncds: Ncds with irreversible chromosome s ilencing (stable Ncds) and secondary recombinants that displayed a gen omic mosaic NotI profile. Segregants from an unstable Ncd population g ave rise to two viable haploid cell types. By contrast, stable Ncds se gregated into a population of viable and inviable haploid cells. We pr opose that the latter are derived from irreversible chromosome silenci ng. Our results indicate that clonal populations of stable Ncds are he terogenous and suggest that segregation and inactivation are independe nt parameters.