Control of virulence gene expression in Francisella tularensis

Dove, Simon

Abstract

Francisella tularensis, the aetiological agent of tularemia, is one of the most infectious bacterial pathogens currently known and a NIAID category A priority pathogen because of its potential for use as a biological weapon. In F. tularensis, the SspA protein family members MglA and SspA play a critical role in regulating the expression of virulence genes that are essential for intramacrophage growth and survival. MglA and SspA form a complex that associates with RNA polymerase (RNAP) to positively control the expression of virulence genes by a mechanism that is not fully understood. We have found that in the live vaccine strain of F. tularensis, the MglA-SspA complex works in concert with the small molecule ppGpp, and a putative DNA-binding protein we have named PigR, to control the expression of a common set of genes. PigR interacts directly with the MglA- SspA complex and ppGpp promotes this interaction. However, it is not known precisely how ppGpp exerts its regulatory effects. We have also uncovered a small sequence motif that is present upstream of the -35 element of regulated promoters that is necessary and sufficient for PigR (and thus presumably MglA, SspA, and ppGpp) to exert its regulatory effects. We refer to this sequence motif operationally as the PigR response element (PRE), but we do not know how this small sequence motif renders a promoter subject to control by these key virulence regulators.
In Aim 1 of this proposal we will test whether ppGpp exerts its effects by binding directly to the MglA-SspA complex, determine the sequence requirements of the PRE, and test whether the PRE serves as a contact site for PigR. RNAP in F. tularensis is highly unusual in that it contains two distinct a-subunits and we have shown that the C-terminal domain of one of the a-subunits (the a2-CTD) specifically influences the expression of MglA-controlled genes.
In Aim 2 we will determine how the a2-CTD exerts its effect on the expression of MglA-controlled genes.
In Aim 3 we will use next-generation sequencing approaches to beter define the MglA regulon and the contribution of its members to intramacrophage growth. The proposed work is expected to reveal how an important intracellular pathogen regulates the expression of genes required for survival in the host, and has implications for how ppGpp and SspA family members regulate gene expression in other pathogenic bacteria. The proposed studies may also help uncover why the transcription machinery in F. tularensis has an unprecedented a-subunit composition. Finally, the proposed work will provide a more complete picture of the contribution of MglA to intramacrophage growth and will also provide an important resource to the Francisella field by defining all of the bacterial requirements for intramacrophage growth.

Public Health Relevance

Francisella tularensis, the causative agent of tularemia, is an intracellular pathogen and a potential bioweapon. Little is known about how genes required for the intracellular growth of Francisella are regulated. We propose to determine how a small molecule, ppGpp, works in concert with the RNA polymerase-associated MglA-SspA complex, and a putative DNA-binding protein named PigR, to positively control the expression of genes that enable Francisella to grow intracellularly. A connection between the activities of ppGpp and regulators of the MglA/SspA class has not previously been appreciated, and may extend to other pathogens where these regulators play important roles in mediating interactions with the host. The proposed experiments are expected to reveal how an important intracellular pathogen regulates the expression of genes that are required to cause disease. Our work has implications for the development of vaccines and small molecules that may protect against tularemia.