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BackgroundFrancisella tularensisis a highly virulent, facultative intracellular pathogen and the etiologic agent of the zoonotic disease Tularemia. RipA is a cytoplasmic membrane protein that is conserved amongFrancisellaspecies and is required for intracellular growth.F. tularensis ripAdeletion mutants escape the phagosome of infected cells, but unlike wild type organisms fail to replicate in the host cell cytoplasm.ResultsFurther analysis ofripAwith respect to environmental effects on the growth of mutant strains and expression levels revealed that RipA is required for optimal growth at pH 7.5 but not pH 6.5. Using a combination of RT-PCR,ripA-lacZtranscriptional and translational fusions, and a RipA-tetracysteine tag fusion protein we found that bothripAtranscription and RipA protein levels were elevated in organisms grown at pH 7.5 as compared to organisms grown at pH 5.5. A number of genes, includingiglA, that are required for intracellular growth are regulated by the transcriptional regulators MglA and SspA, and are induced upon infection of host cells. We quantifiedripAandiglAexpression at different stages of intracellular growth and found that the expression of each increased between 1 and 6 hours post infection. Given the similar intracellular expression patterns ofripAandiglAand that MglA and SspA are positive regulators ofiglAwe tested the impact ofmglAandsspAdeletions onripAandiglAexpression. In the deletion mutant strainsiglAexpression was reduced dramatically as expected, howeverripAexpression was increased over 2-fold.ConclusionExpression ofripAis required for growth at neutral pH, is pH sensitive, and is responsive to the intracellular environment. The intracellular expression pattern ofripAcoincided withiglA, which is positively regulated by MglA and SspA. However, in contrast to their positive impact oniglAexpression, MglA and SspA negatively impactedripAexpressionin vitro.