Radiation Sensitization of Micrococcus radiodurans, Sarcina lutea, and Escherichia coli by p-Hydroxymercuribenzoate

Abstract

The high radioresistance of M. radiodurans results from radical-trapping materials present in the cell as based on studies of sensitization to X-rays, and the binding of p-hydroxymercuribenzoate (HMB) by 3 cell types. Methods include assay of survival by plating techniques, and binding by neutron activation analysis. For a given X-ray dose M. radiodurans tolerates a far higher concentration of HMB, which binds to -SH groups, without a change in survival than do the other cell types. The slope of the survival curve is decreased by HMB more for M. radiodurans than for S. lutea or E. coli B/r. Only bound HMB is effective in sensitizing M. radiodurans, whereas in S. lutea and E. coli B/r loosely bound or free reagent is also effective. The quantity of HMB bound by M. radiodurans is 1.03 [plus or minus] 0.29 x 10-16 mole/cell, far greater than that bound by S lutea or E. coli B/r, 9.45 [plus or minus] 4.10-18 and 1.89 [plus or minus] 0.50 x 10-18 mole/cell respectively, and is consistent with the amount deduced present in extracts of M. radiodurans when compared for protective ability with [beta]-mercaptoe-thylamine. The effective quantity of the cells'' internal radioprotective agent, and thus the amount of HMB bound, are probably lower than these values since only that present in the region of the target would be expected to modify radiation effects.