Stimulation of Growth of an Adenine-requiring Yeast by Purine Analogues

An adenine-requiring yeast grew in an adenine-free medium after considerably long lag period (approximately 170 hr). Supplement of adenine to the medium resulted in a marked reduction of the lag period and in a diauxic growth. Although the growth rate was much faster, a similar diauxic growth was observed in the medium containing sufficient amount of adenine. In both cases, the primary growth occurred as a result of fermentative metabolism of glucose, and after exhaustion of glucose, the secondary growth started at the expense of accumulated ethanol. When cells obtained from the adenine-deficient medium were analyzed for total adenine compounds, approximately six times as much adenine was detected as that amount of adenine added to the medium. Therefore, the yeast has no block in the biochemical sequences leading to adenine biosynthesis but has a defect in the control mechanism of adenine biosynthesis. Adenine appears to initiate adenine biosynthesis under adenine deficient conditions. In order to understand the initiation mechanism by adenine, the effect of several purine analogues on growth was examined. Among the agents tested, 6-thioguanine or 8-azaadenine shortened the lag time of growth in the adenine-free medium. Furthermore, when small amount of adenine was supplied to the medium, these compounds stimulated the primary growth without affecting the secondary growth. On the contrary, 8-azaguanine or 8-azaxanthine markedly stimulated the secondary growth without affecting the primary growth. Thus, two distinct trigger mechanisms in adenine biosynthesis were proposed.