Isolation and characterization of the nucleoid of non-complementing diploids from protoplast fusion in Bacillus subtilis.

Nucleoids of non-complementing diploids (Ncd) from protoplast fusion of B. subtilis were isolated. Their purified DNA banded in neutral CsCl gradient as a single unimodal peak of buoyant density 1.711 g/cm3, a value which is similar to that of the DNA purified from the original parental strains, suggesting that methylation of bases is not a significant factor in chromosome inactivation. Nucleoids released from a Ncd clone give two peaks in a sucrose gradient with a characteristic S value for each nucleoid. That is in contrast to nucleoids from the haploid parents whose sedimental patterns show only one peak. Both nucleoid preparations from Ncd strains assayed for transformation activity show the fast sedimenting nucleoid devoid of transformation activity while the slow nucleoid was active in transformation for the alleles carried by the genome which is expressed in vivo. Both nucleoids of the Ncd strains are transcribed in vivo. The RNA associated with the inactive chromosome is synthesized by the RNA polymerase of the active one. This study provides evidence that inactivation of one parental genome in the Ncd strain may be related with the tertiary organization of its DNA.