# A Versatile Plasmid System for Translational Control and Secretion of Recombinant Proteins in Mycobacteria

## TL;DR

Researchers created a new plasmid system in mycobacteria that allows controlled protein secretion, improving the study of proteins from slow-growing pathogens.

## Contribution

A modular plasmid system combining riboswitches and secretion signals for inducible and efficient recombinant protein secretion in mycobacteria.

## Key findings

- Translationally gated secretion achieved with high extracellular protein levels (10–20x higher than intracellular).
- Theophylline-inducible systems showed dose-dependent response with maximal expression at 2 mM inducer.
- Temperature-sensitive riboswitch (riboU9) demonstrated a clean ON/OFF phenotype triggered by temperature shifts.

## Abstract

Recombinant protein expression in mycobacteria faces
two major
challenges: limited regulatory tools for inducible expression and
inefficient secretion of heterologous products. In this study, we
developed plasmid-based systems that enable translationally gated
secretion in Mycobacterium smegmatis, coupling riboswitch-mediated
translational control with efficient extracellular export. The platform
integrates the M. tuberculosis antigen
85A promoter and signal peptide for constitutive secretion combined
with synthetic riboswitches for inducible translational regulation.
We tested two theophylline-responsive riboswitches (riboE and riboE+)
and a temperature-sensitive variant (riboU9) by using mCherry as a
reporter. Fluorescence assays, RT-PCR, and Western blotting confirmed
efficient secretion and strict translational control. The theophylline-inducible
systems exhibited a dose-dependent response with maximal expression
at 2 mM inducer, while the riboU9 construct showed a clean ON/OFF
phenotype triggered by temperature shift. In all cases, transcripts
were detected irrespective of induction, confirming regulation at
the translational rather than transcriptional level. Secretion was
highly efficient, with 10–20 fold higher protein levels in
extracellular versus intracellular fractions. Induction during early-
and mid-log phases yielded maximal protein, whereas late-log induction
reduced output by ∼50%. Together, these results define translationally
gated secretion as a new control layer in mycobacterial protein production.
This modular platform expands the genetic toolkit available for Mycobacterium
research, providing new opportunities for the study of antigens and
virulence factors from slow-growing pathogens and offering potential
applications in structural biology, vaccine development, and drug
target validation.