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

Noncoding small RNAs are essential for modulating bacterial gene expression, especially under carbon and nutrient-limited conditions. In this study, by employing both in silico and molecular hybridization tools, we identified a carbon source responsive small RNA in A. baumannii DS002. Expression of corresponding gene, abcR200, located at the intergenic region of omt (O-methyltransferase) 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 (P_abcR200). Chromatin immunoprecipitation (ChIP) demonstrated that Lrp oligomers, formed under low leucine conditions, strongly interacted to the P_abcR200. However, the observed interactions were disrupted in the presence of leucine, as leucine promoted dissociation of Lrp to monomers and dimers, the conformation unfavourable to interact with P_abcR200. The abcR200 promoter activity increased with increase of exogenous leucine concentrations, and at 2 mM maximum promoter activity was observed. The AbcR200 target mRNAs were identified by analysing the transcriptome of abcR200 negative strain of A. baumannii. Intriguingly, in abcR200 negative background, expression of lut (lactate utilization) mRNA has increased, suggesting that 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.