Ferritin mutants of Escherichia coli are iron deficient and growth impaired, and fur mutants are iron deficient

Escherichia coli contains at least two iron storage proteins, a ferritin (F tnA) and a bacterioferritin (Bfr). To investigate their specific functions, the corresponding genes (ftnA and bfr) were inactivated by replacing the c hromosomal ftnA and bfr genes with disrupted derivatives containing antibio tic resistance cassettes in place of internal segments of the corresponding coding regions. Single mutants (ftnA::spc and bfr::kan) and a double mutan t (ftnA::spc bfr::kan) were generated and confirmed by Western and Southern blot analyses. The iron contents of the parental strain (W3110) and the bf r mutant increased by 1.5- to 2-fold during the transition from logarithmic to stationary phase in iron-rich media, whereas the iron contents of the f tnA and ftnA bfr mutants remained unchanged. The ftnA and ftnA bfr mutants were growth impaired in iron-deficient media, but this was apparent only af ter the mutant and parental strains had been precultured in iron-rich media . Surprisingly, ferric iron uptake regulation (fur) mutants also had very l ow iron contents (2.5-fold less iron than Fur(+) strains) despite constitut ive expression of the iron acquisition systems. The iron deficiencies of th e ftnA and fur mutants were confirmed by Mossbauer spectroscopy, which furt her showed that the low iron contents of ftnA mutants are due to a lack of magnetically ordered ferric iron clusters likely to correspond to FtnA iron cores. In combination with the fur mutation, ftnA and bfr mutations produc ed an enhanced sensitivity to hydroperoxides, presumably due to an increase in production of "reactive ferrous iron." It is concluded that FtnA acts a s an iron store accommodating up to 50% of the cellular iron during postexp onential growth in iron-rich media and providing a source of iron that part ially compensates for iron deficiency during iron-restricted growth. In add ition to repressing the iron acquisition systems, Fur appears to regulate t he demand for iron, probably by controlling the expression of iron-containi ng proteins. The role of Bfr remains unclear.