Biosynthesis of S-adenosylmethionine by Saccharomyces cerevisiae. I. Adenine and methionine requirments

Uptake and utilization of S-adenosyl-L-methionine and S-adenosyl-L-homocysteine in an adenine mutant of Saccharomyces cerevisiae

An adenine mutant of Saccharomyces cerevisiae was able to utilize S-adenosyl-l-methionine (SAM), S-adenosyl-l-homocysteine (SAH), or adenine as sources for growth and ribonucleic acid (RNA) synthesis. Exogenous SAM or SAH was degraded after entering the cell, and the adenine moiety was reutilized in the endogenous synthesis of the thionium compounds. Part of this endogenous synthesis proceeds from an undetermined de novo system of purine synthesis which contributes significantly to the synthesis of SAM and less so to the RNA purines. Some of the methyl groups of exogenous SAM-methyl-(3)H were incorporated into ribosomal and transfer RNA. Methionine partially antagonized this incorporation. The uptake of SAM-adenine-8-(14)C was not affected by the presence of equal quantities of SAH or adenine in the medium. Exogenous SAM affected the uptake of exogenous SAH-adenine-8-(14)C or adenine-8-(14)C. Exogenous SAH inhibited the uptake of exogenous adenine-8-(14)C.

Effect of L-methionine and S-adenosylmethionine on growth of an adenine mutant of Saccharomyces cerevisiae

A pink, adenine-requiring yeast utilized adenine, hypoxanthine, or S-adenosylmethionine (SAM), in quantities up to 3 mumoles per 100 ml of medium, as equivalent sources of purine for cell growth, but not methylthioadenosine or S-adenosylhomocysteine. Utilization of SAM for growth was inhibited by the presence of l-methionine in quantities greater than 0.6 mumole per 100 ml of medium. However, 6 mumoles of l-methionine had no effect on growth when adenine or hypoxanthine was the source of purine. These sources also reversed the inhibitory effects of 6 mumoles of the amino acid on the utilization of SAM. The presence of 400 mumoles of the amino acid resulted in some inhibition of growth when the organisms were grown with adenine, hypoxanthine, or adenine plus SAM but had no effect on the total uptake of adenine-8-(14)C. Studies on the uptake of radioactivity from a mixture of SAM-adenine-8-(14)C and (3)H-labeled SAM-methyl indicated that these components were taken into the cells at different rates which were altered by the presence of l-methionine. The fixation of (35)S from (35)S-labeled adenosylmethionine into the cells was inhibited by the presence of the amino acid. The cells synthesized and accumulated SAM in the presence of 400 mumoles of l-methionine plus adenine even when exogenous SAM was supplied. Approximately 47% of radioactivity fixed from exogenous SAM-adenine-8-(14)C and 12% from (3)H-labeled SAM-methyl were found in reisolated SAM.