Translational regulation tunes protein expression across the
Toxoplasma gondii life cycle
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peters-michp-phd-bio-2026-thesis .pdf
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Author(s)
Peters, Michelle Lynne
Advisor(s)
Lourido, Sebastian
Date Issued
February 2026
Publisher
Massachusetts Institute of Technology
Abstract
Some of the longest 5′ untranslated regions (UTRs) with the largest numbers of upstream AUGs (uAUGs) documented in eukaryotes belong to parasites of the phylum Apicomplexa. Translational regulation plays prominent roles in the development of these parasites, including the agents of toxoplasmosis (Toxoplasma gondii) and malaria (Plasmodium spp.). However, we lack a global understanding of the functions of these unusual mRNA sequence features in parasite translation. In this thesis, I address this gap by characterizing translation across the acute and chronic stages of the T. gondii asexual life cycle. Chapter 2 describes how I employed high-resolution ribosome profiling of acute-stage T. gondii tachyzoites in human fibroblasts. I show that parasite translation efficiency (TE) is largely controlled by 5′ UTR features and tuned by the number of uAUGs. Unexpectedly, these data also reveal that, despite widespread assembly of parasite monosomes on uAUGs, ribosomes seldom translate upstream open reading frames (uORFs). I reaffirm these determinants of translation through a massively parallel reporter assay examining the effect of more than 30,000 synthetic 5′ UTRs in T. gondii, and implement a machine learning model that accurately predicts the TE of newly designed 5′ UTRs. In Chapter 3, I extend these methods to chronic bradyzoite differentiation. By characterizing early and late timepoints and multiple methods of bradyzoite induction, I reveal critical roles for translational regulation in differentiation that occur specifically in response to environmental cues. I demonstrate that the function of uAUGs in bradyzoites is consistent with our observations in tachyzoites, indicating that uORFs do not tend to function as regulatory switches during differentiation. Together, this work defines the regulatory language of T. gondii translation, providing a framework to
understand the evolution of exceptionally long 5′ UTRs in apicomplexans.
MIT Department
Massachusetts Institute of Technology. Department of Biology
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