Complementation and genetic inactivation: two alternative mechanisms leading to prototrophy in diploid bacterial clones.

Evidence for diploidy at loci located all around the Bacillus subtilis chromosome previously led us to refer to the prototrophic bacterial clones produced by fusion of polyauxotrophic protoplasts as complementing diploid clones (Lévi-Meyrueis et al. 1980; Sanchez-Rivas 1982). In this paper, evidence is presented that gene inactivation may occur in such clones, as judged from the unequal expression of three unselected markers and their low transforming activity in cell lysates, an established property of inactivated genes (Bohin et al. 1982). The insensitivity to protease treatment of the lysates and also the low transforming activity observed with purified DNA may indicate that chromosome inactivation does not necessarily result from the mere attachment of proteins to DNA. Cotransfer by transformation of similarly expressed genes, initially located on separate chromosomes, suggests that genetic recombination has taken place, resulting in the reassortment of active and inactive genes on separate chromosomes. Several genetic structures compatible with the observations are presented which illustrate that prototrophy may result from such reassortment as well as from functional complementation.