MgIA and MgIB are required for the intramacrophage growth of Francisella novicida

Abstract

Francisella novicida is a facultative intracellular pathogen capable of growing in macrophages. A spontaneous mutant of F. novicida defective for growth in macrophages was isolated on LB media containing the chromogenic phosphatase substrates 5-bromo-4-chloro-3-indolyl phosphate (X-p) and designated GB2. Using an in cis complementation strategy, four strains were isolated that are restored for growth in macrophages. A locus isolated from one of these strains complements GB2 for both the intracellular growth defect and the colony morphology on LB (X-p) media. The locus consists of an apparent operon of two genes, designated mgIAB, for macrophage growth locus. Both mgIA and mgIB transposon insertion mutants are defective for intracellular growth and have a phenotype similar to GB2 on LB (X-p) media. Sequencing of mgIA clones from GB2 identified a missense mutation, providing evidence that both mgIA and mgIB are required for the intramacrophage growth of F. novicida. mgIB expression in GB2 was confirmed using antiserum against recombinant MgIB. Cell fractionation studies revealed several differences in the protein profiles of mgI mutants compared with wild-type F. novicida. The deduced amino acid sequences of mgIA and mgIB show similarity to the SspA and SspB proteins of Escherichia coli and Haemophilus spp. in E. coli, SspA and/or SspB influence the levels of multiple proteins under conditions of nutritional stress, and SspA can associate with the RNA polymerase holoenzyme. Taken together, these observations suggest that in Francisella MgIA and MgIB may affect the expression of genes whose products contribute to survival and growth within macrophages.