Double locking of an Escherichia coli promoter by two repressors prevents premature colicin expression and cell lysis

Abstract

SummaryThe synthesis of Eschericha coli colicins is lethal to the producing cell and is repressed during normal growth by the LexA transcription factor, which is the master repressor of the SOS system for repair of DNA damage. Following DNA damage, LexA is inactivated and SOS repair genes are induced immediately, but colicin production is delayed and induced only in terminally damaged cells. The cause of this delay is unknown. Here we identify the global transcription repressor, IscR, as being directly responsible for the delay in colicin K expression during the SOS response, and identify the DNA target for IscR at the colicin K operon promoter. Our results suggest that, IscR stabilizes LexA at the cka promoter after DNA damage thus, preventing its cleavage and inactivation, and this cooperation ensures that suicidal colicin K production is switched on only as a last resort. A similar mechanism operates at the regulatory region of other colicins and, hence, we suggest that many promoters that control the expression of 'lethal' genes are double locked.

The Escherichia coli uropathogenic-specific-protein-associated immunity protein 3 (Imu3) has nucleic acid -binding activity

“…Interestingly, protein-mediated DNA precipitation has been reported in studies describing eukaryotic histones and the E. coli global regulator, protein HU, a known DNA-binding protein [13,14]. Operons, such as those of colicins, that encode proteins that can be detrimental to the producing cell are regulated precisely to ensure appropriate timing of synthesis and avoid untimely death of the producer [15-17]. We can thus speculate that synthesis of Usp and its associated Imu1-3 proteins could also be tightly regulated, limiting their production to avoid overt degradation and masking of the producers’ genome.…”

Regulating colicin synthesis to cope with stress and lethality of colicin production

“…The ssDNA is bound by the RecA protein forming a helical nucleoprotein filament. RecA nucleoprotein filaments at sites of DNA damage activate autocleavage of unbound LexA, provoking dissociation of LexA from SOS boxes to trigger expression of SOS genes [18].…”

“…Binding of two LexA dimers to overlapping LexA boxes is co-operative [21] and the two SOS boxes have been shown to effectively repress expression protecting the producer from overt lysis [22] and inhibit background levels of uninduced transcription [23]. A recent study showed that the association of LexA with a number of SOS operators tested was extremely rapid; however, LexA exhibited variable dissociation rates with significantly slower dissociation from the colicin K activity gene cka [18]. A highly conserved A tract is found in the dual-SOS boxes of bacteriocins at the distal and proximal LexA-binding sites.…”

“…On DNA damage, SOS genes are not all induced at the same time and level. Whereas most are induced almost immediately, colicin-encoding operons are characteristically expressed following a pronounced lag [18,26]. The delay in colicin production has been proposed to provide colicinogenic cells time to repair DNA damage [27].…”

Nutrient-responsive regulation determines biodiversity in a colicin-mediated bacterial community

“…Regulation of the colicin operon is likely to be essential in determining the outcome of colicin-mediated competition, as the conditions under which colicin is produced and released govern the community dynamics. Recent findings suggest a link between nutrient conditions (exploitative competition) and colicin production and release (interference competition): production of colicins 1b, K, and the lysis protein of colicin E7, are controlled by nutrient-responsive regulators [11,32,33]. Environmental factors such as the type and amount of nutrients present in a habitat, may thus have a marked impact on colicin-mediated community dynamics and the persistence of biodiversity.…”