Messenger RNA interferase RelE controls relBE transcription by conditional cooperativity

Abstract

SummaryProkaryotic toxin-antitoxin (TA) loci consist of two genes in an operon that encodes a metabolically stable toxin and an unstable antitoxin. The antitoxin neutralizes its cognate toxin by forming a tight complex with it. In all cases known, the antitoxin autoregulates TA operon transcription by binding to one or more operators in the promoter region while the toxin functions as a co-repressor of transcription. Interestingly, the toxin can also stimulate TA operon transcription. Here we analyse mechanistic aspects of how RelE of Escherichia coli can function both as a co-repressor and as a derepressor of relBE transcription. When RelB was in excess to RelE, two trimeric RelB 2•RelE complexes bound cooperatively to two adjacent operator sites in the relBE promoter region and repressed transcription. In contrast, RelE in excess stimulated relBE transcription and released the RelB2•RelE complex from operator DNA. A mutational analysis of the operator sites showed that RelE in excess counteracted cooperative binding of the RelB2•RelE complexes to the operator sites. Thus, RelE controls relBE transcription by conditional cooperativity.

RETRACTED: Bacterial persistence by RNA endonucleases

“…Using relBE as a model system, we asked if an mRNase encoded by a TA locus could be activated to generate persisters. In exponentially growing cells, RelB is in excess of RelE, and RelE is thus kept inactive (26). We used a nontoxic variant of RelE (called RelE cs6 ) that lacks mRNA cleavage activity but forms a stable complex with RelB to titrate endogenous RelB, and thereby activate endogenous RelE (26).…”

“…The 10 mRNases and their cognate antitoxins are encoded by bicistronic operons that are autoregulated by the antitoxins, which bind to operator sequences in the TA promoter regions (10). The TA complexes bind stronger and cooperatively to the operators; thus, the mRNases themselves function as corepressors of transcription (26). Interestingly, if an mRNase is in excess of its cognate antitoxin, the mRNase destabilizes the promoter-operator complex, and thereby induces TA operon transcription (26).…”

“…The TA complexes bind stronger and cooperatively to the operators; thus, the mRNases themselves function as corepressors of transcription (26). Interestingly, if an mRNase is in excess of its cognate antitoxin, the mRNase destabilizes the promoter-operator complex, and thereby induces TA operon transcription (26). The TA operons of E. coli that encode mRNases are induced by amino acid starvation via a Lon (Long Form Filament)-dependent mechanism (21,27,28).…”

Hypothesis: type I toxin–antitoxin genes enter the persistence field—a feedback mechanism explaining membrane homoeostasis

“…[RelB]) disrupts the RelBE -operator complex by a mechanism that we called 'conditional cooperativity' [30]. Lon protease degrades RelB antitoxin [31].…”

“…RelB is also a transcriptional repressor that, via its ribbon -helix -helix motif, binds to operators in the relBE promoter region [28,29]. Binding of the RelBE complex to the operators is highly cooperative and represses the relBE promoter very efficiently [30]. Excess RelE ([RelE] .…”