Acts as a positive regulator of sigma-B activity in response to salt and heat stress by stimulating the activity of the RsbT kinase toward RsbS in vitro. One of 4 functionally non-identical RsbR paralogs, it functions in the environmental signaling branch of the general stress response. Negative regulator of sigma-B activity. Non-phosphorylated RsbS binds to RsbT, preventing its association with RsbU. Requires any one of RsbRA, RsbRB, RsbRC or RsbRD to sequester RsbT. When RsbS and the RsbR paralog(s) are phosphorylated, they release RsbT, which can then bind and activate RsbU. Interacts with RsbRB and RsbS in the stressosome. The stressosome probably also contains RsbRC and RsbRD. Phosphorylated by RsbT. This threonine phosphorylation abrogates the ability of RsbRA to stimulate RsbT in vitro. Cells lacking this gene have a decreased response to salt stress, indicating that RsbRA is a positive regulator of sigma-B activity. Its activity is dependent on RsbRB.