Adaptation of AmtR-controlled gene expression by modulation of AmtR binding activity in Corynebacterium glutamicum

In corynebacteria, nitrogen regulation is controlled by the TetR family protein AmtR, which was extensively studied in the last years. In frame of these studies a number of AmtR binding sites were identified and characterized and it became obvious that for distinct genes the number and sequences of these sites varied significantly. In this study, the influence of numbers and alterations of AmtR binding sites were addressed by in vivo and in vitro studies. It can be concluded that in general a single highly conserved AmtR site is sufficient for stringent regulation and that non-conserved binding sites have a very limited influence, despite the fact that binding of AmtR was shown for several of these sites, e.g. upstream of amtA, amtB and gdh. Furthermore, the reason for and consequences of the lack of AmtR autoregulation were addressed in vivo. The introduction of a spacing nucleotide between the two conserved half sites of the AmtR binding box alone is sufficient to restore AmtR autoregulation. The main differences observed between wild type and an AmtR autoregulation strain were a slightly enhanced background of transcription of AmtR-controlled genes and a slightly slower response to nitrogen limitation.