A ComGA-dependent checkpoint limits growth during the escape from competence

In Bacillus subtilis, competence for transformation develops in 5-10% of th e cells in a stationary phase culture. These cells exhibit a prolonged lag in the resumption of growth and cell division during the escape from compet ence. To better understand the basis of this lag, we have characterized com petent cultures microscopically. To distinguish the minority of competent c ells, a translational fusion between ComK, the competence transcription fac tor, and the green fluorescent protein (GFP) was used as a marker. Only 5-1 0% of the cells in a competent culture were fluorescent, indicating that Co mK synthesis is an all or nothing event. To validate the identification of competent cells, we demonstrated the coincident expression of comEA, a late competence gene, and comK-gfp. Competent cells resemble stationary phase c ells; the majority are single (not in chains), contain single nucleoids, an d rarely contain FtsZ rings. Upon dilution into fresh medium, competent cel ls maintain this appearance for about 2 h, in contrast, the majority of non -competent cells rapidly resume growth, exhibiting chaining, nuclear divisi on and FtsZ-ring formation. The late competence protein ComGA is required f or the competence-related block in chromosome replication and cell division . In the competent cells of a comGA mutant culture, chromosomal replication and FtsZ-ring formation were no longer blocked, although competent comGA m utant cells were abnormal in appearance. It is likely that one role for Com GA is to prevent growth, chromosome replication and cell division until Com K can be eliminated by degradation. A mutation in the ATP-binding site of c omGA inactivated the protein for transformation but did not prevent it from inhibiting DNA replication and cell division. The buoyant density differen ce between competent end non-competent cells depends on the competence-spec ific growth arrest.