Strains of Escherichia coli containing the Nif+ plasmid pRD1 were used to investigate the possibility that haem proteins are involved in the regulation by O2 of nif expression. Strains lacking 5-aminolaevulinate synthase (HemA-), and hence normally unable to synthesize haem proteins, showed an identical response to O2 in the presence or absence of added aminolaevulinate (and hence of haem proteins). It was concluded that the regulatory protein NifL is not a haem protein.
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Smith et al. (1988) studied this question.
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