DECODING TOXICITY: CHARACTERIZATION OF THE IBSC/SIBC TYPE I TOXIN-ANTITOXIN SYSTEM OF ESCHERICHIA COLI

Abstract

<p>Small RNAs and small proteins encoded in diverse microbial genomes have been shown to play big parts in regulatory processes that are vital to the defence, adaptability, and survival of their hosts despite their small size. These elements also make up the two components of type I toxin-antitoxin (TA) systems that are present across many bacterial lineages. In these TA systems, the production of small toxic peptides can disrupt cellular processes and induce growth attenuation. However, toxin synthesis can be antagonized by its cognate RNA antitoxin, which binds toxin transcripts in order to inhibit their translation. As the functions of some of these TA pairs emerge, it has become evident that their presence is beneficial to the cell, taking on roles such as the guardians of genomic integrity or the coordinators of cellular responses. At present, the biological function and regulation of many type I TA pairs remain enigmatic. Among them is the IbsC/SibC pair found in <em>Escherichia coli</em>. In this dissertation, our efforts in identifying and characterizing IbsC/SibC are discussed. After coming across <em>ibsC/sibC</em> through a genetic screen and confirming that the production of the IbsC peptide is toxic to cells, we examined the sequence requirements for the toxicity of IbsC. We further probed into the effects of mutations on its structure and mechanism of action. In investigating the regulation of <em>ibsC </em>expression, we found that toxin production is tightly repressed under nutrient rich conditions at the transcriptional and post-transcriptional levels, the latter of which is mediated by the SibC RNA. Furthermore, we took advantage of the toxicity of IbsC by engineering an efficient molecular cloning system with its variants using the sequence information garnered from our prior studies. Taken together, our work has shed light on the biochemical properties, evolution, and potential applications of this TA pair.</p>