SUMMARY: Mutant Mir M7 of exhibits a pH-dependent morphological interconversion; growing cells are rod-shaped at pH 5.5 but round at pH 7. Round cells cannot divide or make DNA, RNA and proteins at pH 5.5. At this pH, cell division and DNA synthesis are resumed either when substances such as Mg, Na, sucrose or spermine are added to the growth medium or immediately after the appearance of a normal rod morphology. Round cells, but not rods or the wild type, are lysed by low concentrations of detergents and are killed by DL-amino acids. Their growth rate is not improved by vigorous aeration and they make spheroplasts at an incubation temperature of 8 °C. This behaviour is the opposite of that of rods and the original strain. The transition from rods to round cells at pH 7 does not occur when cell division is inhibited by penicillin. Round cells must divide several times in an acid medium to give regular and uniform rods.

The possibility of a different state of the membrane in round and rod-shaped cells and a dependence of cell division, DNA synthesis, and morphology on the state of the membrane are discussed.


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