Aurantimycin A

The bacterium Listeria monocytogenes ubiquitously occurs in the environment but can cause severe invasive disease in susceptible individuals when ingested. We recently identified the L. monocytogenes genes lieAB and lftRS, encoding a multidrug resistance ABC transporter and a regulatory module, respectively. These genes jointly mediate resistance against aurantimycin, an antibiotic produced by the soil-dwelling species Streptomyces aurantiacus, and thus contribute to the survival of L. monocytogenes in its natural habitat, the soil. Repression of lieAB and lftRS is exceptionally tight but strongly induced in the presence of aurantimycin. Repression depends on LftR, which belongs to subfamily 2 of the PadR-like transcriptional repressors. To better understand this interesting class of transcriptional repressors, we here deduce the LftR operator sequence from a systematic truncation and mutation analysis of the P lieAB promoter. The sequence identified is also present in the P lftRS promoter but not found elsewhere in the chromosome. Mutational analysis of the putative operator in the P lftRS promoter confirmed its relevance for LftR-dependent repression. The proposed operator sequence was sufficient for DNA binding by LftR in vitro, and a mutation in this sequence affected aurantimycin resistance. Our results provide further insights into the transcriptional adaptation of an important human pathogen to survive the conditions in its natural reservoir.IMPORTANCEListeria monocytogenes is an environmental bacterium that lives in the soil but can infect humans upon ingestion, and this can lead to severe invasive disease. Adaptation to these entirely different habitats involves massive reprogramming of transcription. Among the differentially expressed genes is the lieAB operon, which encodes a transporter for the detoxification of aurantimycin, an antimicrobial compound produced by soil-dwelling competitors. While lieAB is important for survival in the environment, its expression is detrimental during infection. We here identify critical elements in the lieAB promoter and its transcriptional regulator LftR that contribute to habitat-specific expression of the lieAB genes. These results further clarify the molecular mechanisms underlying the aurantimycin resistance of L. monocytogenes.

Bacteria can cope with toxic compounds such as antibiotics by inducing genes for their detoxification. A common detoxification strategy is compound excretion by ATP-binding cassette (ABC) transporters, which are synthesized upon compound contact. We previously identified the multidrug resistance ABC transporter LieAB in Listeria monocytogenes, a Gram-positive bacterium that occurs ubiquitously in the environment, but also causes severe infections in humans upon ingestion. Expression of the lieAB genes is strongly induced in cells lacking the PadR-type transcriptional repressor LftR, but compounds leading to relief of this repression in wild-type cells were not known. Using RNA-Seq and promoter-lacZ fusions, we demonstrate highly specific repression of the lieAB and lftRS promoters through LftR. Screening of a natural compound library yielded the depsipeptide aurantimycin A - synthesized by the soil-dwelling Streptomyces aurantiacus - as the first known naturally occurring inducer of lieAB expression. Genetic and phenotypic experiments concordantly show that aurantimycin A is a substrate of the LieAB transporter and thus, lftRS and lieAB represent the first known genetic module conferring and regulating aurantimycin A resistance. Collectively, these genes may support the survival of L. monocytogenes when it comes into contact with antibiotic-producing bacteria in the soil.

The Gram-positive bacterium Listeria monocytogenes occurs ubiquitously in the environment and infects humans upon ingestion. It encodes four PadR-like repressors, out of which LftR has been characterized previously and was shown to control gene expression in response to the antibiotic aurantimycin produced by other environmental bacteria. To better understand the PadR regulons of L. monocytogenes, we performed RNA-sequencing with mutants of the other three repressors LadR, LstR and Lmo0599. We show that LadR is primarily responsible for the regulation of the mdrL gene, encoding an efflux pump, while LstR and Lmo0599 mainly regulate their own operons. The lstR operon contains the lmo0421 gene, encoding a homolog of the RodA/FtsW protein family. However, this protein does not possess such functionality, as we demonstrate here. The lmo0599 operon contains two additional genes coding for the hypothetical trans-membrane proteins lmo0600 and lmo0601. A striking phenotype of the lmo0599 mutant is its impaired growth at refrigeration temperature. In light of these and other results we suggest that Lmo0599 should be renamed and propose LltR (listerial low temperature regulator) as its new designation. Based on the nature of the PadR target genes we assume that these repressors collectively respond to compounds acting on the cellular envelope.