Absence of functional RNA encoded by a silent chromosome in non-complementing diploids obtained from protoplast fusion in Bacillus subtilis.

The molecular basis for lack of phenotypic expression of one chromosome in Bacillus subtilis non-complementing diploid clones (Ncd cells) was investigated. Correlations between chromosomal inactivation and absence of functional transcripts were determined with wild-type prophage phi 105 or a thermoinducible mutant phi 105 cts23, on either the expressed or the silent chromosome. It appears that no significant amount of phage mRNA is detectable in Ncd cells carrying the prophage in the inactive chromosome. However, phi 105 mRNA represents 0.23% of total cellular mRNA in an Ncd strain with the prophage in the expressed chromosome and 0.28% in the parental lysogenic strain. The lack of an mRNA repressor of phi 105 prophage from the silent chromosome was confirmed by the absence of repressor activity in Ncd clones with a temperature sensitive mutant phi 105 located in the silent chromosome. After heat induction, no phage production was observed. As expected these clones do not exhibit phi 105 immunity when superinfected with the same phage. The combined data of the present and previous work suggest that control of phenotypic suppression of Ncd strains should, at the transcription level, involve a different DNA tertiary organisation in one of the two chromosomes.