A variety of DNA-glycosylases have been described that remove "abnormal" purines and pyrimidines from DNA leaving a deoxyribosyl residue (AP-site) in the polynucleotide chain. These AP-sites are also formed spontaneously from certain labile modified purines in DNA. Our research is concerned with the biological properties of this DNA lesion and is designed to answer the following questions: Whether AP-sites (1) produce mutations; (2) if they do, what kind(s) of mutation (base substitution, frameshift, large deletion, insertion or rearrangement) are produced; (3) what role, if any, do DNA repair systems play in producing these mutations. Our experimental system utilizes bacteriophage phi X174. Our approach consists of 4 parts: (1) Synthesis of phi X RF DNA carrying an AP-site at a single preselected site in phi X gene G (position 2402 of the minus strand). A combination of chemical and enzymatic steps is used to achieve this site-specific modification. (2) Examination of the site-modified DNA in vivo by transfection of spheroplasts derived from various E. coli mutants that are defective in certain DNA repair systems. (3) Isolation and pro]agation of any mutant virus produced from the transfection experiments. This is accomplished with phi X-sensitive host cells containing a plasmid carrying a functional copy of phi X gene G. These host cells are permissive for base-substitution, frameshift, large deletion or insertion mutations in phi X gene G. (4) Identification of the mutation by sequencing the region of gene G in mutant DNA that carried the site-specific modification (AP-site) in the parental DNA. We believe this approach will provide a direct answer to the questions posed above. It is hoped that this information will extend our understanding of the molecular mechanisms of mutation by AP-sites because they are a major DNA lesion both spontaneoiusly and under conditions of treatment with chemical carcinogens.