RT Journal Article SR Electronic(1) A1 Acuña, Lillian G. A1 Barros, M. José A1 Peñaloza, Diego A1 Rodas, Paula I. A1 Paredes-Sabja, Daniel A1 Fuentes, Juan A. A1 Gil, Fernando A1 Calderón, Iván L.YR 2016 T1 A feed-forward loop between SroC and MgrR small RNAs modulates the expression of eptB and the susceptibility to polymyxin B in Salmonella Typhimurium JF Microbiology, VO 162 IS 11 SP 1996 OP 2004 DO https://doi.org/10.1099/mic.0.000365 PB Microbiology Society, SN 1465-2080, AB Base-pairing small RNAs (sRNAs) regulate gene expression commonly by direct interaction with cognate mRNAs. Nevertheless, recent studies have expanded this knowledge with the discovery of the RNA ‘sponges’ which are able to interact and repress the functions of classical base-pairing sRNAs. In this work, we present evidence indicating that the sponge RNA SroC from Salmonella enterica serovar Typhimurium base pairs with the MgrR sRNA, thereby antagonizing its regulatory effects on both gene expression and resistance to the antimicrobial peptide polymyxin B (PMB). By a predictive algorithm, we determined putative SroC–MgrR base-pairing regions flanking the interaction area between MgrR and its target mRNA, eptB, encoding a LPS-modifying enzyme. With a two-plasmid system and compensatory mutations, we confirmed that SroC directly interacts and down-regulates the levels of MgrR, thus relieving the MgrR-mediated repression of eptB mRNA. Since it was previously shown that an Escherichia coli strain carrying an mgrR deletion is more resistant to PMB, we assessed the significance of SroC in the susceptibility of S. Typhimurium to PMB. Whereas the sroC deletion increased the sensitivity to PMB, as compared to the wild-type, the resistance phenotypes between the ΔmgrR and ΔsroCΔmgrR strains were comparable, evidencing that mgrR mutation is epistatic to the sroC mutation. Together, these results indicate that both SroC and MgrR sRNAs compose a coherent feed-forward loop controlling the eptB expression and hence the LPS modification in S. Typhimurium., UL https://www.microbiologyresearch.org/content/journal/micro/10.1099/mic.0.000365