Porphyrin-Accumulating Mutants of Escherichia coli

Abstract

Four mutants ( pop-1, pop-6, pop-10 , and pop-14 ) which accumulate a red water-insoluble pigment were obtained in Escherichia coli K-12 AB1621. For each mutant, the red pigment was shown to be protoporphyrin IX, a late precursor of heme. Mutagenic treatment of mutant pop-1 yielded a secondary mutant, pop-1 sec-20 , which accumulated a brown water-soluble pigment. The brown pigment was shown to be coproporphyrin III. Mutant pop-1 resembled the parental strain in its cytochrome absorption spectrum, catalase activity, and ability to grow on nonfermentable carbon and energy sources; therefore, its ability to produce and utilize heme was unimpaired. Judged on the same criteria, the secondary mutant, pop-1 sec-20 , was partially heme and respiratory deficient. Growth in anaerobic conditions decreased by 25% the accumulation of protoporphyrin by pop-1; under the same conditions, pop-1 sec-20 did not accumulate coproporphyrin or coproporphyrinogen. The mutations causing protoporphyrin accumulation in all four pop mutants were found to map in the lac to purE (10–13 min) region of the E. coli chromosome. In the case of mutant pop-1 , the mutation was shown to be strongly linked to the tsx locus (12 min). In mutant pop-1 sec-20 , the second mutation causing coproporphyrin accumulation was co-transducible with the gal locus at a frequency of 88 to 96%. The mechanism of porphyrin accumulation by the mutants is discussed.