The deoxyoligonucleotide dTTAGCAGAACCGG is complementary to a region of yeast iso-1 cytochrome c m-RNA; hence this oligonucleotide serves as a hybridization probe for detecting the yeast cyc 1 gene, which codes for iso-1 cytochrome c. In eco RI restriction-gel electrophoresis-hybridization experiments by the method of E.M. Southern, we have shown that P32 dTTAGCAGAACCGG hybridizes with an eco RI restriction fragment which is absent from a cyc 1 deletion strain. Using this information, we will isolate and clone the cyc 1 gene and surrounding sequences, using an appropriate safe vector to propage the recombinant genome in E. coli. Subsequent to cloning it, the cyc 1 gene sequence will be used to study regulation of cyc 1 m-RNA synthesis in vivo (as a hybridization probe) and in vitro (as a DNA template for transcription experiments). BIBLIOGRAPHIC REFERENCES: Andrew, C., Hopper, A. K., and B. D. Hall (1976). A Yeast Mutant Defective in the Processing of 27S r-RNA Precursor. Molec. Gen. Genetics 144, 29-37. Shultz, L. D. and B. D. Hall (1976). Transcription in yeast: alpha-Aminitin sensitivity and other properties which distinguish between RNA polymerases I and III. Proc. Nat. Acad. Sci. 73, 1029.