Compartmentalized transcription and the establishment of cell type during sporulation in Bacillus subtilis

Abstract

An early step in sporulation of the bacterium Bacillus subtilis, is the formation of two compartments in the developing sporangium: the mother cell and the forespore. These compartments differ in their programs of gene expression and developmental fate. The establishment of cell type within this simple developmental program, is accomplished by the compartmentalization of sigma subunits of RNA polymerase. The localization of these sigma factors results in compartment-specific gene expression. Recent experiments have elucidated some of the early steps in the establishment of cell type. After septum formation, the activity of the sigma factor, sigma F, is confined to the forespore compartment. This, in turn, results in the localized expression of another developmental sigma factor, sigma G. The forespore localization of these two sigma factors, establishes the forespore line of gene expression. sigma F and sigma G also regulate mother cell events. sigma F activity in the forespore regulates the proteolytic processing of sigma E within the mother cell compartment. The localization sigma E activity leads to mother cell expression of another sigma factor, pro-sigma K. The proteolytic processing of pro-sigma K to mature sigma K is controlled by the forespore sigma factor, sigma G. Mature sigma K then directs the transcription of mother cell specific genes. Therefore, the initial localization of sigma F activity to the forespore compartment, orchestrates the establishment of cell type in both forespore and mother cell compartments.