Cessation of Respiration after Far-Ultraviolet Irradiation of Escherichia coli B/r: Loss of Unaltered Pyridine Nucleotides to the Medium

Cessation of respiration of Escherichia coli B/r cells is initiated 30 min after irradiation at 254 nm and is linked to cell death. Pyridine nucleotides begin to disappear with the onset of respiratory failure and are almost completely absent from the cells by 90 min after irradiation. We studied the fate of these respiratory cofactors in a niacin-requiring mutant (RSI) grown on minimal medium containing $[7-{}^{14}{\rm C}]$ nicotinic acid. By 90 min after irradiation ($52\ {\rm J}/{\rm m}^{2}$) nearly all of the acid-soluble radioactive counts appeared in the medium. Paper chromatographic studies and a spectrophotometric assay indicated that the material was nicotinamide adenine dinucleotide and nicotinamide adenine dinucleotide phosphate. The loss of nicotinamide adenine dinucleotide was not balanced by synthesis, despite the presence of appropriate active biosynthetic enzymes for at least 90 min after uv irradiation. Analysis of the amino acid and nucleotide pool of the cells showed that there was some loss of most of these small molecules; the levels of a few were almost completely depleted. We conclude that the pyridine nucleotides are lost from the cell to the medium and that the loss cannot be attributed to extensive general membrane damage.