The distribution and diversity of nucleoside N-deoxyribosyltransferase among lactic acid bacteria
Nucleoside N-deoxyribosyltransferases are capable of synthesising stereoand regioselectively a variety of 2'-deoxy- and 2', 3'-dideoxyribonucleosides. These compounds could be effective as inhibitors of reverse transcriptase, by acting as DNA chain terminators. The enzyme has been found previously only in a small number of microorganisms of the genus Lactobacillus. In the present study, phosphate-independent nucleoside N-deoxyribosyltransferase activity was detected in cell-free extracts from the genera Aerococcus, Pediococcus, LeuconostocS, treptococcus, Lactococcus and also more widely among the subgroups of Lactobacillus. From this screening study, it was found that only purine-specific N-deoxyribosyltransferase activity was present in Lactobacillus salivarius subsp. salivarius, suggesting that this strain lacked an enzyme analogous to N-deoxyribosyltransf erase II. Leucollostoc mesenteroides subsp. cremoris was found to express the highest N-deoxyribosyltransferase specific activity towards pyrimidines and purines. As N-deoxyribosyltransferase activity had not been detected in strains of Leuconostocb efore, the enzyme was purified and characterised from Leu. mesenteroidess ubsp. cremoris. A single multifunctional enzyme capable of carrying out the transfer of the deoxyribosyl moiety from either pyrimidine or purine nucleosides to either pyrimidine or purine bases was found to be present in this strain of Leucoizostoc.A. four-step procedure employing affinity chromatography was used to purify the enzyme to homogeneity. Kinetic studies carried out on the purified enzyme showed that the transfer reactions ocurred via a ping-pong mechanism. Evidence was also provided by radiolabelling studies of a glycosyl-enzyme intermediate forming when the first substrate, the nucleoside donor, bound to the enzyme.