Conversion of the bifunctional 8-oxoguanine/beta-delta apurinic/apyrimidinic DNA repair activities of Drosophila ribosomal protein S3 into the human S3 monofunctional P-elimination catalyst through a single amino acid change

The Drosophila 53 ribosomal protein has important roles in both protein tra nslation and DNA repair. In regards to the latter activity, it has been sho wn that 53 contains vigorous N-glycosylase activity for the removal of 8-ox oguanine residues in DNA that leaves baseless sites in their places. Drosop hila 53 also possesses an apurinic/apyrimidinic (AP) lyase activity in whic h the enzyme catalyzes a beta -elimination reaction that cleaves phosphodie ster bonds 3' and adjacent to an AP lesion in DNA. In certain situations, t his is followed by a delta -elimination reaction that ultimately leads to t he formation of a single nucleotide gap in DNA bordered by 5' and 3'-phosph ate groups. The human 53 protein, although 80% identical to its Drosophila homolog and shorter by only two amino acids, has only marginal N-glycosylas e activity. Its lyase activity only cleaves AP DNA by a beta -elimination r eaction, thus further distinguishing itself from the Drosophila 53 protein in lacking a delta -elimination activity. Using a hidden Markov model analy sis based on the crystal structures of several DNA repair proteins, the enz ymatic differences between Drosophila and human 53 were suggested by the ab sence of a conserved glutamine residue in human 53 that usually resides at the cleft of the deduced active site pocket of DNA glycosylases. Here we sh ow that the replacement of the Drosophila glutamine by an alanine residue l eads to the complete loss of glycosylase activity. Unexpectedly, the delta -elimination reaction at AP sites was also abrogated by a change in the Dro sophila glutamine residue. Thus, a single amino acid change converted the D rosophila activity into one that is similar to that possessed by the human 53 protein. In support of this were experiments executed in vivo that showe d that human 53 and the Drosophila site-directed glutamine-changed 53 perfo rmed poorly when compared with Drosophila wildtype 53 and its ability to pr otect a bacterial mutant from the harmful effects of DNA-damaging agents.