Subcellular localization of proteins governing the proteolytic activation of a developmental transcription factor in Bacillus subtilis

Background: Spore formation in Bacillus subtilis takes place in a sporangium consisting of two compartments called the forespore and the mother cell, Late in development, when the forespore is wholly contained within the mother cell, gene transcription is coordinated between the compartments by an intercellular signal transduction pathway, This pathway operates at the level of proteolytic processing of the proprotein precursor (pro-sigma(K)) to the mother-cell transcription factor sigma(K). The conversion of pro-sigma(K) to sigma(K) is governed by the putative processing enzyme SpoIVFB and its negative regulator SpoIVFA, which are produced in the mother cell. Results: We used fluorescence microscopy in conjunction with antibodies against SpoIVFA and SpoIVFA and a fusion of SpoIVFB to the Green Fluorescent Protein from Aquorea victoria to visualize these proteins in the sporangium. Both proteins were found to co-localize with the forespore region of the sporangium, a finding consistent with the idea that SpoIVFA and SpoIVFB, which are inferred to be integral membrane proteins, are located in the mother cell membrane that surrounds the forespore. Conclusions: We conclude that SpoIVFA and SpoIVFB are situated at the boundary between the forespore and the mother cell, at which location SpoIVFB could be activated by a signalling protein produced in the forespore.