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Evidence is presented that near-ultraviolet (near-UV) light can alter galactoside transport in Escherichia coli in several independent ways. It can inactivate the permease system per se, it can interfere with metabolic energy production or transfer, and it can cause an increase in the generalized permeability of the membrane. Earlier publications suggested that near-UV destroys cofactors needed for electron transport and thus places a limitation on energy reserves. In agreement, we found that the active accumulation of [14C]thiomethyl-beta-D-galactopyranoside is decreased after irradiation by a larger factor than that due to action directly on the permease system. The effect on the latter was measured by the decrease in the rate of o-nitrophenyl-beta-D-galactopyranoside (ONPG) transport. As evidence that energy supplies for this "downhill" process did not become rate limiting after irradiation, we found that carbonylcyanide-m-chlorophenyl-hydrazone did not stimulate ONPG transport of irradiated cells. Cells genetically deficient in functional permease or cells treated with formaldehyde still transport ONPG passively, although at much lower rates. With the use of such cells, it was found that high fluences (doses) made the cells leaky. Further evidence that the permease system and the metabolic energy system can be inactivated independently is also presented. It is shown that a photoproduct from the irradiation of chloramphenicol inactivates the permease system much more efficiently than the energy system. In addition, it is shown that thio-beta-D-digalactopyranoside protects the permease system, but not the energy system, both against direct inactivation by near-UV and against photosensitized inactivation in the presence of chloramphenicol.