Friends of the Ribosome: Translational Regulation in Mycobacteria

Abstract

Mycobacteria are responsible for a significant disease burden in the world, caused by Mycobacterium tuberculosis (Mtb) and non-tuberculous mycobacteria (NTM). While curable with a number of well-established antibiotic regimes, the global impact of mycobacterial disease is worsening due to increasing antimicrobial resistance. Understanding how this genus regulates its growth, causes disease, and tolerates antibiotics is necessary for the development of new antibiotics. The mycobacterial ribosome is a viable drug target, not only because of its central role in protein synthesis, but also because of its unique properties as a bacterial ribosome. Work on mycobacterial translational regulation has already revealed a number of features of protein synthesis machinery unique to mycobacteria. We were interested in understanding mycobacterial translational control through the lens of known ribosome-associated factors. In Chapter 2, we characterize the role of LepA, a mysterious, conserved translational GTPase, in mycobacterial permeability. We found that in Mycobacterium smegmatis (Msm), LepA controls the synthesis of a family of mycobacterial porins, and this control maintains cellular permeability. Here, translational regulation during protein synthesis is used to maintain the cellular proteome, specifically the most abundant constituents of the mycobacterial membrane. In Chapter 3, we conducted a biochemical screen to find protein associations with the mycobacterial ribosome. In log-phase, we identified mycobacterial-specific secretion systems (ESX-3 and ESX-5) that have not been previously associated with the ribosome. Our investigation of ribosome-associated factors, and their role in translational regulation, has led us to the mycobacterial membrane. We found a translational GTPase that helps to synthesize outer membrane proteins in Msm, and we found a significant proportion of ribosomes associated with a number of secretion systems. This leads us to propose a model whereby mycobacteria, and likely other bacteria require a heterogeneous population of ribosomes in a single cell. For mycobacteria, who have a complex cell-wall architechture, it may be that a significant proportion of ribosomes are dedicated to membrane translation and protein export. We believe that this connection, between protein synthesis and the mycobacterial membrane, may be an Achilles’ heel for pathogenic mycobacteria, where novel drug targets could exist.