# A leucine responsive small RNA AbcR200 regulates expression of the lactate utilization (lut) operon in Acinetobacter baumannii DS002

## TL;DR

This paper identifies a small RNA, AbcR200, in Acinetobacter baumannii that regulates lactate utilization genes in response to leucine levels.

## Contribution

The study reveals a novel leucine-responsive small RNA, AbcR200, that regulates lactate utilization through translational inhibition of lut mRNA.

## Key findings

- AbcR200 is regulated by leucine via the Lrp transcription factor, with promoter activity peaking at 2 mM leucine.
- The lut mRNA is a target of AbcR200, with sequence complementarity and elevated protein levels in abcR200 negative strains.
- Lrp interacts with the AbcR200 promoter under low leucine conditions, but this interaction is disrupted in the presence of leucine.

## Abstract

Noncoding small RNAs are essential for modulating bacterial gene expression, especially under carbon and nutrient-limited conditions. In this study, by using both in silico and molecular hybridization tools, we identified a carbon source responsive small RNA in Acinetobacter baumannii DS002. Expression of corresponding gene, abcR200, located at the intergenic region of omt (O-methyl transferase) and orf72 genes, is under the transcriptional control of a global transcriptional factor, leucine responsive regulatory protein (Lrp). A sequence motif that serves as a target for Lrp was found overlapping the abcR200 promoter (PabcR200). Chromatin immunoprecipitation demonstrated that Lrp oligomers, formed under low leucine conditions, strongly interacted to the PabcR200. However, the observed interactions were disrupted in the presence of leucine, as leucine promoted dissociation of Lrp to monomers and dimers, the conformation unfavorable to interact with PabcR200. The abcR200 promoter activity increased with increase of exogenous leucine concentrations, and at 2 mM leucine concentration, maximum promoter activity was observed. The AbcR200 target mRNAs were identified by analyzing the transcriptome of abcR200 negative strain of A. baumannii. Intriguingly, in abcR200 negative background, expression of lut (lactate utilization) mRNA has increased, suggesting lut mRNA as one of the mRNA targets for AbcR200. Consistent of this observation, there existed extensive sequence complementarity between AbcR200 and lut mRNA, especially in the regions coding LutP, LutE, and LutR. In support of the observed sequence complementarity, the levels of lut mRNA encoded proteins got elevated in abcR200 negative HS002 strains suggesting a role for AbcR200 in translational inhibition of lut mRNA.