TI - Strain -dependent occurrence of functional GTP : AMP phosphotransferase (AK3) in Saccharomyces cerevisiae . AB - The gene for yeast GTP : AMP phosphotransferase (PAK3) was found to encode a nonfunctional protein in 10 laboratory strains and one brewers' strain . The protein product showed high similarity to vertebrate AK3 and was located exclusively in the mitochondrial matrix . The deduced amino acid sequence revealed a protein that was shorter at the carboxyl terminus than all other known adenylate kinases . Introduction of a +1 frameshift into the 3'-terminal region of the gene extended homology of the deduced amino acid sequence to other members of the adenylate kinase family including vertebrate AK3 . Frameshift mutations obtained after in vitro and in vivo mutagenesis were capable of complementing the adk1 temperature-conditional deficiency in Escherichia coli , indicating that the frameshift led to the expression of a protein that could phosphorylate AMP . Some yeasts , however , including strain D273-10B , two wine yeasts , and two distantly related yeast genera , harbored an active allele , named AKY3 , which contained a +1 frameshift close to the carboxyl terminus as compared with the laboratory strains . The encoded protein exhibited GTP : AMP and ITP : AMP phosphotransferase activities but did not accept ATP as phosphate donor . Although single copy in the haploid genome , disruption of the AKY3 allele displayed no phenotype , excluding the possibility that laboratory and brewers' strains had collected second site suppressors . It must be concluded that yeast mitochondria can completely dispense with GTP : AMP phosphotransferase activity .