CONTROL OF TRANSCRIPTION OF THE BACILLUS-SUBTILIS SPOIIIG GENE, WHICH CODES FOR THE FORESPORE-SPECIFIC TRANSCRIPTION FACTOR-SIGMA-G

The Bacillus subtilis spoIIG gene codes for a sigma factor termed sigma-G which directs transcription of genes expressed only in the forespore compartment of the sporulating cell. Use of spoIIIG-lacZ transcriptional fusions showed that spoIIIG is cotranscribed with the spoIIG operon beginning at t0.5-1 of sporulation. However, this large mRNA produced little if any sigma-G, and transferring the spoIIIG gene without the spoIIG promoter into the amyE locus resulted in a Spo+ phenotype. Significant translation of spoIIIG began at t2.5-3 with use of an mRNA whose 5' end is just upstream of the spoIIIG coding sequence. Synthesis of this spoIIIG-specific mRNA was not abolished by a deletion in spoIIIG itself. Similar results were obtained when a spoIIIG-lacZ translational fusion lacking the spoIIG promoter was integrated at the amyE locus. These data suggest that synthesis of sigma-G is dependent neither on transcription from the spoIIG promoter nor on sigma-G itself but can be due to another transcription factor. This transcription factor may be sigma-F, the product of the spoIIAC locus, since a spoIIAC mutation blocked spoIIIG expression, and sequences upstream of the 5' end of the spoIIIG-specific mRNA agree well with the recognition sequence for sigma-F. RNA polymerase containing sigma-F (E-sigma-E) initiated transcription in vitro on a spoIIIG template at the 5' end found in vivo, as did E-sigma-G. However, E-sigma-F showed a > 20-fold preference for spoIIIG over a known sigma-G-dependent gene compared with the activity of E-sigma-G.