This sequence is also a preferential DNR-intercalating site where

This sequence is also a preferential DNR-intercalating site where a mutually exclusive competitive binding of DNR and DnrN occurs. This may be the mechanism that senses the intracellular DNR level to either turn on or turn off the expression of DnrI, which is the key activator for DNR biosynthesis. This study shows the circular nature of regulation, where three elements namely the DnrI activator, the DrrA–DrrB efflux pump and DNR are acting in sequence. At a steady-state level of antibiotic production, DnrI activates the drrA–drrB operon as

well as major biosynthetic operons. The efflux system maintains the intracellular DNR at an optimum concentration, and a micro increase in the intracellular DNR level leads to preferential intercalation at the DnrN-binding PKC412 supplier site that shuts down dnrI transcription temporarily. The intercalated drug must leave the site before DnrN can bind and reactivate dnrI, which is possibly affected by DrrC (Lomovskaya et al., 1996). Yet another regulation is by the control of DnrN expression, which is dictated by its activator DnrO that binds at the upstream element near the dnrN promoter. This site is also a preferential intercalating site for DNR (Otten et al., 2000). These combined factors possibly fine tune the feedback regulation of drug biosynthesis. We analyzed the effect of the drrAB mutation on the three regulatory genes dnrN, dnrO and dnrI along with the structural gene dpsA, which is essential for polyketide biosynthesis (Grimm et al., 1994). qRT-PCR results show that both dnrI and dpsA are downregulated to 1/8th and 1/16th, respectively,

when compared with the WT (Fig. 4b). The melting-curve analysis shows a single peak for the respective amplicons and the amplification efficiency plot had a slope <0.1 (Fig. 4a). This finding confirms the hypothesis that an increase in the DNR level is sensed and the key activator of drug biosynthesis DnrI is downregulated. This results in a decline of dpsA expression, which is essential for polyketide biosynthesis. In the null mutant, DnrN has to failed to activate dnrI transcription in spite of a 2.2-fold increase in the dnrN transcript relative to WT as seen in qRT-PCR results. The DnrN-binding site at the dnrI promoter region is a high-affinity site for DNR intercalation (Furuya & Hutchinson, 1996). Therefore, a small increase in the DNR level within the cell is sufficient to exclude DnrN from its activation site. It is intriguing that dnrN/O has an upstream element that is intercalated by DNR in competition with DnrO, which is an activator protein of dnrN transcription (Otten et al., 2000). The possible reason for the increase in the dnrN transcript is that DnrO possibly binds to a second activation site indicated in a previous report (Jiang & Hutchinson, 2006). Nevertheless, the slight increase in the dnrN transcript in the mutant remains unexplained. qRT-PCR shows that the DnrO transcript level increases by 3.4-fold in the mutant relative to WT.

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