¹ From the Department of Bacteriology, University of California, Berkeley

Endogenous photosensitization in a carotenoidless mutant of Rhodopseudomonas spheroides has been studied.

When this mutant is exposed to visible light and oxygen there is a short lag period followed by exponential killing of the cells. The killing process obeys the Bunsen-Roscoe law of reciprocity and is temperature-independent.

The chlorophyll content of the cells does not affect the rate of killing until a certain low threshold has been reached.

Exposure of photosynthetically grown cells to air in the dark induces a temporary desensitization to the killing.

A certain proportion of the population is invariably resistant to the photokilling. Evidence is presented to show that the resistance of these cells is probably a result of their abnormally low chlorophyll content.

One consequence of the irradiation is the appearance of abnormally small colonies upon plating the survivors. This is interpreted in terms of a multi-hit theory.

The photosensitizing pigment itself (the intracellular bacteriochlorophyll) is destroyed. The process is zero order, temperature-dependent, and does not follow the reciprocity law.

It is suggested that the death of the cell results from a disruption of the cell membrane.

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