Regulation of toxinogenesis in Corynebacterium diphtheriae: mutations in the bacterial genome that alter the effects of iron on toxin production

Abstract

Mutants of Corynebacterium diphtheriae C7(beta) that are resistant to the inhibitory effects of iron on toxinogenesis were identified by their ability to form colonies surrounded by toxin-antitoxin halos on agar medium containing both antitoxin and a high concentration of iron. Chromosomal mutations were essential for the altered phenotypes of four independently isolated mutant strains. During growth in deferrated liquid medium containing various amounts of added iron, these mutants differed from wild-type C. diphtheriae C7(beta) in several ways. Their growth rates were slower under low-iron conditions and were stimulated to various degrees under high-iron conditions. The concentrations of iron at which optimal toxin production occurred were higher for the mutants than for wild-type C. diphtheriae C7(beta). Toxin production by the mutants during growth in low-iron medium occurred throughout the period of exponential growth at nearly constant rates that were proportional to the bacterial growth rates. In contrast, toxin production by wild-type C. diphtheriae C7(beta) in similar low-iron cultures occurred predominantly during the late exponential phase, when iron was a growth-limiting nutrient. Additional studies demonstrated that these mutants had severe defects in their transport systems for ferric iron. We propose that the altered regulation of toxinogenesis by iron in our mutants was caused by the severe defects in their iron transport systems. As a consequence, the mutants exhibited a low-iron phenotype during growth under conditions that permitted wild-type C. diphtheriae C7(beta) to exhibit a high-iron phenotype.