A spontaneous chromosomal amplification of the ADH2 gene in Saccharomyces cerevisiae

A spontaneous antimycin A-resistant mutant carrying approximately four extra copies of ADH2 on chromosome XII was isolated from yeast strain 315-1D which lacks a functional copy of ADH1 and thus is antimycin A-sensitive. The additional copies of the normally glucose-repressed ADH2 are expressed during growth on glucose accounting for the antimycin A resistance. These extra copies are inserted into nonadjacent ribosomal DNA sequences (rDNA) near the recombination stimulating sequence HOT1. Each extra copy of the ADH2 gene (1548 bp) replaces most of the 37S transcript (approximately 7400 bp) in one of the approximately 200 copies of the rDNA present in the yeast genome. All four extra copies of ADH2 are lost at a rate of approximately 1 x 10(-5) deletions per cell per generation. One of the joints between the rDNA and ADH2 DNA is located 7 nucleotides downstream from 20 adenine residues in the normal copy of ADH2. This joint occurs at the end of a stretch of 16-29 thymidines in the rDNA which has been expanded to 57-59 thymidines. The other novel joint is located in a short region of sequence similarity between ADH2 and the rDNA. These observations suggest that amplification of ADH2 was a two step process: first the ADH2 gene was inserted into the rDNA, then multiple copies were generated by unequal crossing over or gene conversion within the rDNA.