Sigma factors are initiation factors that promote the attachment of RNA polymerase to specific initiation sites and are then released. This sigma factor is the primary sigma factor during exponential growth. Its activity is stimulated by RbpA. Homotrimer (Potential). interacts transiently with the RNA polymerase core complex. Interacts with RNA polymerase-binding protein RbpA via its sigma-2 region (residues 211-347) in a free form. Belongs to the sigma-70 factor family. Truncated N-terminus.