Phenotypic properties of a large-cell, radiation-resistant strain of Escherichia coli

Mean cell volumes, total amount of deoxyribonucleic acid (DNA), number of nuclear bodies, and X-ray responses were determined for exponential cultures of Escherichia coli P6, a large-cell radiation-resistant mutant strain, and for E. coli 82/r, the parent strain of P6, growing in three different media at varying growth rates. Combined results comparing cultures in the same growth medium indicated that exponentially growing P6 cells are 2,5 ± 0.2 times larger, contain 3.6 ± 0.3 times more DNA, and 1.8 ± 0.1 times more nuclear bodies than comparable 82/r cells. Individual P6 nuclear bodies contained 2.01 + 0.13 times as much DNA as 82/r nuclear bodies in comparable cultures, suggesting that each P6 nuclear body may contain two complete 82/r genomes. All P6 cultures gave sigmoidal X-ray survival curves with extrapolation numbers averaging 2.0 and with limiting slopes smaller in absolute value than the slopes of the exponential survival curves obtained for 82/r cultures. P6 cells appeared to be no more efficient than 82/r cells in enzymatic dark repair of X-ray or ultraviolet damage, suggesting that the greater X-ray resistance of P6 cells may be associated with the doubled DNA content of each P6 nuclear body.