Patterns of Expression and Translocation of the Ubiquitin Ligase SlrP in Salmonella enterica Serovar Typhimurium

SlrP is an E3 ubiquitin ligase that can be translocated into eukaryotic host cells by the two type III secretion systems that are expressed by Salmonella enterica serovar Typhimurium and are encoded in Salmonella pathogenicity islands 1 (SPI1) and 2 (SPI2). Expression of slrP and translocation of its product were examined using lac, 3 x FLAG, and cyaA' translational fusions. Although slrP was expressed in different media, optimal expression was found under conditions that imitate the intravacuolar environment and promote synthesis of the SPI2-encoded type III secretion system. Translocation into mammalian cells took place through the SPI1- or the SPI2-encoded type III secretion system, depending on specific host cell type and timing. A search for genetic factors involved in controlling the expression of slrP unveiled LeuO, Lon, and the two-component system PhoQ/PhoP as novel regulators of slrP. Our experiments suggest that LeuO and Lon act through HilD under SPI1-inducing conditions, whereas PhoP directly interacts with the slrP promoter to activate transcription under SPI2 inducing conditions.