Heidi Contreras

Mycobacterium tuberculosis has evolved various mechanisms by which the bacterium can maintain homeostasis under numerous environmental assaults generated by the host immune response. M. tuberculosis harbors enzymes involved in the oxidative stress response that aid in survival during the production of reactive oxygen species in activated macrophages. Previous studies have shown that a dye-decolorizing peroxidase (DyP) is encapsulated by a bacterial nanocompartment, encapsulin (Enc), whereby packaged DyP interacts with Enc via a unique C-terminal extension. M. tuberculosis also harbors an encapsulin homolog (CFP-29, Mt-Enc), within an operon with M. tuberculosis DyP (Mt-DyP), which contains a C-terminal extension. Together these observations suggest that Mt-DyP interacts with Mt-Enc. Furthermore, it has been suggested that DyPs may function as either a heme-dependent peroxidase or a deferrochelatase. Like Mt-DyP, M. tuberculosis iron storage ferritin protein, Mt-BfrB, and an M. tuberculosis protein involved in folate biosynthesis, 7,8-dihydroneopterin aldolase (Mt-FolB), have C-terminal tails that could also interact with Mt-Enc. For the first time, we show by co-purification and electron microscopy that mycobacteria via Mt-Enc can encapsulate Mt-DyP, Mt-BfrB, and Mt-FolB. Functional studies of free or encapsulated proteins demonstrate that they retain their enzymatic activity within the Mt-Enc nanocompartment. Mt-DyP, Mt-FolB, and Mt-BfrB all have antioxidant properties, suggesting that if these proteins are encapsulated by Mt-Enc, then this nanocage may play a role in the M. tuberculosis oxidative stress response. This report provides initial structural and biochemical clues regarding the molecular mechanisms that utilize compartmentalization by which the mycobacterial cell may aid in detoxification of the local environment to ensure long term survival.

Mycobacterium tuberculosis protein pairs Rv1246c-Rv1247c, Rv2865-Rv2866, and Rv3357-Rv3358, here named RelBE, RelFG, and RelJK, respectively, were identified based on homology to the Escherichia coli RelBE toxin:antitoxin (TA) module. In this study, we have characterized each Rel protein pair and have established that they are functional TA modules. Overexpression of individual M. tuberculosis rel toxin genes relE, relG, and relK induced growth arrest in Mycobacterium smegmatis; a phenotype that was completely reversible by expression of their cognate antitoxin genes, relB, relF, and relJ, respectively. We also provide evidence that RelB and RelE interact directly, both in vitro and in vivo. Analysis of the genetic organization and regulation established that relBE, relFG, and relJK form bicistronic operons that are cotranscribed and autoregulated, in a manner unlike typical TA modules. RelB and RelF act as transcriptional activators, inducing expression of their respective promoters. However, RelBE, RelFG, and RelJK (together) repress expression to basal levels of activity, while RelJ represses promoter activity altogether. Finally, we have determined that all six rel genes are expressed in broth-grown M. tuberculosis, whereas relE, relF, and relK are expressed during infection of human macrophages. This is the first demonstration of M. tuberculosis expressing TA modules in broth culture and during infection of human macrophages.

Modified Vaccinia Ankara (MVA) is a highly attenuated poxvirus vector that is widely used to develop vaccines for infectious diseases and cancer. We demonstrate the construction of a vaccine platform based on a unique three-plasmid system to efficiently generate recombinant MVA vectors from chemically synthesized DNA. In response to the ongoing global pandemic caused by SARS coronavirus-2 (SARS-CoV-2), we use this vaccine platform to rapidly produce fully synthetic MVA (sMVA) vectors co-expressing SARS-CoV-2 spike and nucleocapsid antigens, two immunodominant antigens implicated in protective immunity. We show that mice immunized with these sMVA vectors develop robust SARS-CoV-2 antigen-specific humoral and cellular immune responses, including potent neutralizing antibodies. These results demonstrate the potential of a vaccine platform based on synthetic DNA to efficiently generate recombinant MVA vectors and to rapidly develop a multi-antigenic poxvirus-based SARS-CoV-2 vaccine candidate.