# Heterologous Substitution of Mycobacterium tuberculosis rRNA in Mycobacterium smegmatis and Its Impact on Antimicrobial Susceptibility

## TL;DR

Researchers replaced rRNA in a fast-growing mycobacterium with that from a tuberculosis-causing bacterium, finding it grew slower and showed reduced antibiotic sensitivity.

## Contribution

A novel method using serine integrase-mediated recombination to substitute rRNA operons in mycobacteria was developed and tested.

## Key findings

- The BRkoA strain showed significantly slower growth compared to the wild-type strain.
- BRkoA exhibited reduced susceptibility to ribosome-targeting antibiotics like kanamycin and linezolid.
- Cryo-EM revealed missing ribosomal proteins in the BRkoA strain's 70S ribosome.

## Abstract

Background: The global incidence of multidrug-resistant and extensively drug-resistant tuberculosis continues to rise. The ribosome serves as a target for multiple antimicrobials, making functional research on it hold great significance. Methods: Using homologous recombination combined with a multiple serine integrase-mediated site-specific recombination system, we replaced the two endogenous rRNA operons in Mycobacterium smegmatis MC2 155 with a single copy of the single rRNA operon from Mycobacterium tuberculosis H37Rv, constructing the M. smegmatis BRkoA strain. We assessed growth kinetics at 37 °C, cold sensitivity at lower temperatures, transcriptional levels by RT-qPCR, 70S ribosome integrity through cryo-EM, and antimicrobial susceptibility by microdilution assays. Results: The BRkoA strain was successfully constructed. It exhibited markedly slower growth compared to the wild-type strain. Cold-sensitivity assays indicated potential ribosome assembly defects, while transcriptional analysis suggested altered rRNA processing and modification. Cryo-EM analysis further demonstrated the absence of specific ribosomal proteins in the BRkoA 70S ribosome. Moreover, BRkoA displayed reduced susceptibility tendency to several ribosome-targeting antibiotics, including kanamycin, amikacin, paromomycin, gentamicin, and linezolid. Conclusions: Replacement of the two endogenous rrn operons in M. smegmatis with a single copy of the single M. tuberculosis rrn operon using a serine integrase-mediated recombination system caused growth impairment and decreased sensitivity tendency to several ribosome-targeting antimicrobials. These findings suggest that ribosome structural variation contributes to intrinsic drug resistance mechanisms.