Distinctive Sulphydryl Responses in X-Irradiated and in Heat-treated Micrococcus radiodurans

Abstract

THE role of endogenous sulphydryl compounds (SH) as partial determinants of radio-resistance has been challenged in wild type and in radio-sensitive mutants of Micrococcus radiodurans and in cell-free radio-protective extracts prepared from the wild strain^1,2. While agents such as p-hydroxymercuribenzoate (HMB) and iodoacetic acid (IAA) complex cellular sulphydryl groups and sensitize this cell to ionizing radiation^3,4, it has been shown that both HMB and IAA suffer radiolytic decomposition and possibly poison the cell by a non-sulphydryl vector^5,6. When HMB was used to titrate the endogenous SH of eleven bacterial strains, however, a linear relationship was discovered between the quantity bound and both the LD₉₀ and D₀ exposures⁷. Recent studies reveal (a) that M. radiodurans binds more HMB than expected from its larger mass, (b) that the change in HMB binding after both X-irradiation and heat exposure is similar, and (c) that this response differs from responses of more radio-sensitive bacteria such as Escherichia coli B/r and Sarcina lutea.