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The expression of the Pasteurella multocida toxin (PMT) gene toxA was investigated. Growth in vitro at 30 °C or added iron caused less than 4-fold repression of toxA expression. The putative repressor TxaR was expressed in Escherichia coli but deletion and frameshift mutations abolishing TxaR production had no effect on toxA expression. Naturally occurring nontoxigenic mutants which contained the toxA gene had no large rearrangements near toxA or changes in toxA promoter structure. Thus PMT is constitutively expressed and is only regulated in a minor way.
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Hoskins et al. (1996) studied this question.
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