We would like to completely define the nucleotide sequence of a viral DNA, and then to understand the functional significance of each nucleotide in this sequence. In order to understand the sequence we plan to develop techniques for altering it at will by chemical and enzymatic procedures. We also proposed to develop general and practical methods for the construction of mutants containing any desired alteration of known DNA sequence. We hope to apply such methods to construct mutants in regions of the phi X174 genome where sequence is known but genetic function is still a mystery. We hope to apply our methods for the construction of altered DNA sequences to the analysis of other bacterial genetic material, and eventually to eukaryotic genetic elements propagated in bacteria. During the coming year we plan: 1. To develop a method for retrieving oligonucleotides from sequenced DNA, and to use them as mutagens. 2. To maintain an up to date DNA sequence data base. 3. To pursue the use of base analogs as specific mutagens. 4. To precisely map Xenopus promoters and terminators on sequenced DNAs.