(3) have shown the arrest of synthesis of macromolecules to be a consequence of the lowering of cellular ATP: in uncA mutants, which lack the Mg2+,Ca2+-activated ATPase (ATP phosphohydrolase, EC 3.6.1.3), the colicin failed to lower ATP levels and synthesis of macromolecules continued. Both transport of proline and of glutamine, apparently coupled to different forms of energy (4), were uncoupled by colicin K in the uncA mutants.Since synthesis of macromolecules continued in colicintreated uncA cells, it was paradoxical that they were not viable. At low multiplicities of colicin, two or three killing units per cell, growth and cell division should have generated daughter cells free of colicin, which then would produce colonies. Death might have resulted from an effect of the colicin not reversible in the daughter cells, or from direct inhibition of cell division. Alternatively, the inability of colicin-treated cells to concentrate metabolites or ions such as K+ and Mg2+ (5,6) Preparation of Colicins. Strain K235 (Col K) was induced with mitomycin C and the colicin purified according to Kunugita and Matsuhashi (8). Both fractions eluted from the CM-Sephadex column were used with similar results; each showed several protein bands upon polyacrylamide gel electrophoresis. Colicins El, E2, E3, and Ib were also induced with 0.2 mg of mitomycin C per liter. The colicinogenic bacteria were washed with 0.05 M sodium phosphate at pH 7.0, resuspended in the same buffer, and broken by sonication. The crude extracts were dialyzed against the same buffer and stored frozen.Assay of ATP. Samples (0.1 ml) of a bacterial suspension were added to 1.9 ml of boiling water and boiled for 15 min. One milliliter of the extract was added to 2 ml of 0.05 M potassium arsenate, pH 7.4, and 0.02 M MgSO4. Firefly lantern extract (Sigma), 0.05 ml, was added and the emitted light counted in a Packard model 314-EX2 liquid scintillation counter using the "analyser" mode. RESULTSConditional viability of colicin-treated uncA cells High ATP levels and synthesis of macromolecules are maintained in colicin-treated uncA cells in spite of the uncoupling of active transport (3). In the K+ phosphate-buffered medium employed, however, high levels of intracellular K+ would have remained. Inability to retain K+ after transfer to NaCl-based solid medium could have prevented the colicintreated cells from forming colonies. Fig. 1 shows that a large fraction of colicin-treated uncA cells can survive to form colonies on LBK agar, in which KC1 has replaced the usual NaCl. The optimal pH of LBK for survival of colicin-treated cells was 6.6; half as many colonies appeared at pH 6.0 or 7.1 (data not shown).

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Mutants in three genes affecting two Mg2+ transport systems are described. System I, for which C02+, Mn2+, and Mg2+ are substrates, is inactive in corA mutants. corB mutants express system I after growth on high (10 mM) Mg2+ but not low (0.1 mM) Mg2+. Both corA and corB mutants are resistant to C02+ or Mn2. corA mutants are sensitive to Ca2. Transport system II is specific for Mg2+ and is repressed by growth on 10 mM Mg2+. mgt mutations inactivate system II. Growth of mgt mutants is normal except on very low (1 ,uM) concentrations of Mg2+. corA mgt strains exhibit no high-affinity, energy-dependent transport of Mg2+ and require 10 mM Mg2+ for optimal growth. The three genes are not linked. The corA locus is cotransducible with ilv at 75 min, corB is cotransducible with pyrB at 85 min, and mgt is cotransducible with malB and mel at 81 min on the genetic map.

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