Anti-sigma factor for SigI. Negatively regulates SigI activity through direct interaction. Interacts (via RsgI N-terminal anti-sigma domain) with SigI. Part of the sigI-rsgI operon, which is transiently induced by heat stress. Expression is positively regulated by SigI via the sigma-I promoter (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 RsgI (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, however cell diameter is slightly decreased.