Cells of E. coli B/r and${\rm B}_{{\rm s}-1}$ in exponential growth and labeled with14 C were analyzed for phospholipid composition after irradiation with either γ rays or uv radiation and reincubation for 0 to 180 min. Radiation doses ranged from 1 to 36 krad for${\rm B}_{{\rm s}-1}$, and either 3 to 54 krad γ- or 500 to$4000\ {\rm ergs}/{\rm mm}^{2}$ of uv-irradiation for B/r. Phospholipid composition was measured in terms of either14 C activity or phosphorus content. In unirradiated cells it was phosphatidylethanolamine (PE): 75-82%: phosphatidylglycerol (PG): 14-21%; cardiolipin (CL): 2-5%. The most significant change with time postirradiation was an increase in the total amount of CL and CL%, both of which were strain and dose dependent. In the γ-dose ranges studied, CL% at 180 min postirradiation increased continuously in B/r (14.2% at 54 krad), while it reached a plateau at 9 krad in${\rm B}_{{\rm s}-1}$ (9.0%). The amounts of CL present showed a four- to fivefold increase over control levels. After$4000\ {\rm ergs}/{\rm mm}^{2}$ uv, the CL% of B/r only rose to 6% and the amount of CL was double that in control cells. After γ doses, up to 9% of the recovered lipid activity was present in a nonphospholipid fraction of unidentified composition.

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