Complex sporulation-specific expression of transcription termination factor Rho highlights its involvement in Bacillus subtilis cell differentiation

Transcription termination factor Rho controls pervasive, mainly antisense, transcription initiated at cryptic signals or resulting from read-through at weak terminators in various bacterial species. In Bacillus subtilis, Rho is intricately involved in the regulation of phenomena associated with the adaptation to stationary phase and cell differentiation including the ultimate survival program of sporulation. While knockout or overexpression of the rho gene alters global transcription and modifies cell physiology, in wild-type B. subtilis cells, the reduction of Rho levels during the transition to stationary phase is necessary for both initiation and implementation of the sporulation program. However, the mechanisms that govern Rho expression throughout the cell cycle remain largely unknown. Here, we demonstrate that, besides the previously identified vegetative SigA-dependent promoter active during exponential growth, two distinct mechanisms ensure a spatiotemporal expression of the rho gene during sporulation. In the mother cell of the sporangium, rho expression occurs through the read-through transcription initiated at the distal SigH-dependent and Spo0A[~]P-regulated promoter of the spo0F gene. In the forespore, rho is transcribed from a genuine promoter recognized by the alternative sigma factor SigF. These regulatory elements compensate for the inactivation of SigA-dependent rho expression at the end of exponential growth and allow the critical "refueling" of Rho protein in both compartments of the sporangium. We show that altering rho expression in the mother cell or in the forespore affects differently the properties and the morphology of mature spores. Moreover, spores formed in the absence of Rho are impaired in their ability to revive under favorable growth conditions, exhibiting accelerated germination and slow outgrowth. Finally, we show that optimal outgrowth of the wild-type spores requires the expression of rho during spore maturation and additionally after spore germination.