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Toxin
is synthesized in high yield after the supply of Fe++ and Fe+++ has become exhausted.
Remarkably, C. diphtheriae will synthesize diphtheria toxin as 5% of its total
protein, when starved for iron. The tox gene is
regulated by a mechanism of negative control wherein a repressor molecule, product of the
DtxR gene, is activated by iron. The active repressor binds to the tox gene operator and
prevents transcription. When iron is removed from the repressor (under growth conditions
of iron limitation), derepression occurs, the repressor is inactivated and transcription
of the tox genes can occur. Iron is referred to as a corepressor since it is required for
repression of the toxin gene. |
