Nick Pokorzynski, PhD

Bacterial pathogens must successfully adapt to changing environments during infection as the host attempts to limit their proliferation. My research therefore aims to understand the mechanistic basis for bacterial adaptation, typically by focusing on the regulation of gene expression and the activity of the regulated gene products.

During my PhD training in the laboratory of Dr. Rey Carabeo, I developed a model for how the obligate intracellular bacterial pathogen, Chlamydia trachomatis, integrates its response to the deprivation two essential nutrients, iron and tryptophan. We demonstrated that C. trachomatis regulates the expression of their tryptophan salvage pathway in a unique manner through a novel iron-dependent transcription factor, YtgR. C. trachomatis salvage tryptophan from the precursor indole, provided by the microflora of the female lower genital tract. This environment is also notoriously iron-limited. Thus, YtgR regulation of tryptophan salvage ties the availability of indole to the level of iron in the environment, presumably enabling the proliferation of the pathogen. Interestingly, YtgR is regulated not only by iron levels, but also by proteolytic cleavage from an N-terminal permease domain. We observed that this domain was enriched for tryptophan codons, and contained a rare motif of three sequential tryptophan codons (WWW). Our subsequent work demonstrated that YtgR levels were regulated in a tryptophan-dependent manner via the WWW motif, conferring an attenuation-like effect to the expression of the tryptophan salvage pathway. Thus, the WWW motif completes a regulatory feedback loop, linking YtgR abundance to the availability of tryptophan, the biosynthesis of which it regulates. Chlamydia therefore appears to have evolved an exceedingly unique strategy to moderate the expression of the tryptophan salvage pathway, which likely reflects the unique environment they have adapted to in the female lower genital tract.

After defending my thesis, I continued in the Carabeo lab as a postdoctoral associate where I compared the chlamydial and host cell transcriptional response to nutritional stress to identify instances of host-pathogen conflict during persistent infections. We demonstrated that a nutritionally-stressed infected host cell is not necessarily compromised in its ability to control an infection. In fact, we found that nutrient-starved host cells can deploy alternative, secondary nutritional stresses to further limit pathogen growth. Both the primary nutritional stress and the secondary host amplification of anti-bacterial stress work synergistically to control the infection. Thus, the response of the pathogen to a given nutritional stress is the combined result of both the primary insult and the response of the host cell to the same insult.

My postdoctoral appointment in the laboratory of Dr. Eduardo Groisman at the Yale School of Medicine is focused on how Salmonella enterica coordinates central metabolic demands with the adverse nutritional conditions associated with infection. When Salmonella infects a mammalian cell, it is starved of essential nutrients, and this signal(s) induces virulence programs. Both the nutritional stress and the induction of virulence slow Salmonella growth. How does this alter the metabolism of an intracellular pathogen, and what regulates this process? We aim to elucidate the metabolic prioritization that takes place during infection to uncover new determinants of bacterial virulence.