A mutant of Escherichia coli K 12 has been isolated affected in a gene, designated cydD, distinct from the three previously described loci involved in the synthesis of assembly of the cytochrome bd oxidase complex. The mutant, obtained by nitrosoguanidine mutagenesis, lacks the spectroscopically detectable components of this oxidase, namely cytochromes bSs8, bsg5 and d . Cytochrome oxidase o is the sole CO-binding cytochrome in membranes of the mutant, but the soluble haemoprotein b-590 and catalase activity appear unaffected. Discrimination between Cyd+ and Cyd-strains is facilitated by the development of a defined low-phosphate medium that allows the inclusion of Zn2+ as well as azide, inhibitors of respiratory electron transfer particularly via cytochrome 0. Mapping with F-prime factors and by P 1 cotransductional frequencies shows the mutation to map near 19.3 min on the E. coli chromosome, distinct from cydC, which maps at 18.9 min. The gene order in this region was tested in a three-factor cross and demonstrates the order zbj : : TnlO(YYC 199)-cydD-aroA, consistent with cotransduction frequencies.
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