Nonconventional splicing of the gene encoding the Hac1p transcription activator regulates the unfolded protein response (UPR) in Saccharomyces cerevisiae. This simple on/off switch contrasts with a more complex circuitry in higher eukaryotes. Here we show that a heretofore unrecognized pathway operates in yeast to regulate the transcription of HAC1. The resulting increase in Hac1p production, combined with the production or activation of a putative UPR modulatory factor, is necessary to qualitatively modify the cellular response in order to survive the inducing conditions. This parallel endoplasmic reticulum-to-nucleus signaling pathway thereby serves to modify the UPR-driven transcriptional program. The results suggest a surprising conservation among all eukaryotes of the ways by which the elements of the UPR signaling circuit are connected. We show that by adding an additional signaling element to the basic UPR circuit, a simple switch is transformed into a complex response.

Many pathogens regulate or modify their immune-stimulating ligands to avoid detection by their infected hosts. Listeria monocytogenes, a facultative intracellular bacterial pathogen, interacts with multiple components of mammalian innate immunity during its infection cycle. During replication within the cytosol of infected cells, L. monocytogenes utilizes two multidrug efflux pumps, MdrM and MdrT, to secrete the small nucleic acid second messenger cyclic-di-AMP (c-di-AMP). Host recognition of c-di-AMP triggers the production of type I interferons, including beta interferon (IFN-β), which, surprisingly, promote L. monocytogenes virulence. In this study, we have examined the capacity of multiple laboratory and clinical isolates of L. monocytogenes to stimulate host production of IFN-β. We have identified the L. monocytogenes strain LO28 as able to hyperinduce IFN-β production in infected cells ∼30-fold more than the common laboratory clone L. monocytogenes strain 10403S. Genomic analyses determined that LO28 contains a naturally occurring loss-of-function allele of the transcriptional regulator BrtA and correspondingly derepresses expression of MdrT. Surprisingly, while derepression of MdrT resulted in hyperstimulation of IFN-β, it results in significant attenuation in multiple mouse models of infection. While type I interferons may promote L. monocytogenes virulence, this study demonstrates that unregulated expression of the c-di-AMP-secreting efflux pump MdrT significantly restricts virulence in vivo by an unknown mechanism.

How the innate immune system tailors specific responses to diverse microbial infections is not well understood. Cells use a limited number of host receptors and signaling pathways to both discriminate among extracellular and intracellular microbes, and also to generate responses commensurate to each threat. Here, we have addressed these questions by using DNA microarrays to monitor the macrophage transcriptional response to the intracellular bacterial pathogen Listeria monocytogenes. By utilizing combinations of host and bacterial mutants, we have defined the host transcriptional responses to vacuolar and cytosolic bacteria. These compartment-specific host responses induced significantly different sets of target genes, despite activating similar transcription factors. Vacuolar signaling was entirely MyD88-dependent, and induced the transcription of pro-inflammatory cytokines. The IRF3-dependent cytosolic response induced a distinct set of target genes, including IFNbeta. Many of these cytosolic response genes were induced by secreted cytokines, so we further identified those host genes induced independent of secondary signaling. The host response to cytosolic bacteria was reconstituted by the cytosolic delivery of L. monocytogenes genomic DNA, but we observed an amplification of this response by NOD2 signaling in response to MDP. Correspondingly, the induction of IFNbeta was reduced in nod2-/- macrophages during infection with either L. monocytogenes or Mycobacterium tuberculosis. Combinatorial control of IFNbeta induction by recognition of both DNA and MDP may highlight a mechanism by which the innate immune system integrates the responses to multiple ligands presented in the cytosol by intracellular pathogens.