Use of integrational plasmid excision to identify cellular localization of gene expression during sporulation in Bacillus subtilis.

Sporulation in Bacillus subtilis is a simple developmental system involving the differentiation of two sister cells, the prespore and the mother cell. Many of the genes that regulate sporulation (spo genes) are thought to be expressed differentially. However, direct demonstration of differential gene expression, by fractionation of prespore and mother cell proteins, is possible only at a relatively late stage of development. H. De Lencastre and P. J. Piggot (J. Gen. Microbiol. 114:377-389, 1979) have described a genetic method for determining the cellular location of the requirement for spo gene expression. Here we describe a similar method based on the use of integrational plasmids that can insertionally inactivate any given spo gene. Loss of the integrated plasmid by homologous recombination leads to the restoration of spo gene function. If this occurs just before sporulation begins, the phenotypes of the progeny of heat-resistant spores should depend on whether the gene is required in the prespore or the mother cell. Thus, we show that for known prespore-specific genes, such as spoIIIG and spoVA, only phenotypically Spo+ progeny that have lost the integrated plasmid are produced. In contrast, for mother-cell-specific genes, such as spoIIIC and spoVJ, a substantial proportion of the progeny are asporogenous, having retained the integrated plasmid. On the basis of our results, the spoIID and spoIIIA genes, which are expressed soon after division, appear to be required only in the mother cell compartment.