Sigma factors are initiation factors that promote the attachment of RNA polymerase to specific initiation sites and are then released. This sigma factor is involved in regulation of cell wall metabolism in response to heat stress. Acts by regulating the expression of genes such as bcrC, mreBH and lytE (PubMed:18156261, PubMed:21541672, PubMed:23199363). Also plays a role in survival at low temperatures (PubMed:19114499). Negatively regulated by the anti-sigma-I factor RsgI. Upon exposure to heat, SigI is released from RsgI and activated. Transient heat activation of SigI may depend upon DnaK chaperone. Interacts with RsgI. Part of the sigI-rsgI operon, which is transiently induced by heat stress. Expression is positively autoregulated via the sigma-I promoter (PubMed:11157964, PubMed:17185538). In exponentially growing cells, expression is regulated by the WalRK two-component system, which represses the sigma-I promoter and activates the sigma-A promoter, leading to the formation of a basal level of SigI (PubMed:23199363). WalRK can also positively and directly regulate transcription of the operon under heat stress through a binding site located upstream of the sigma-I promoter (PubMed:24125693). Repressed by glucose (PubMed:17185538). Cells lacking this gene exhibit normal cell morphology and growth rate at 37 degrees Celsius, but cannot grow at high temperature. Belongs to the sigma-70 factor family. SigI subfamily.