Sporulation in Saccharomyces cerevisiae: Premeiotic DNA synthesis, readiness and commitment

Sporulation and meiosis in yeast are studied under conditions of a relatively high degree of synchrony. This is achieved by the use of strains selected for their high sporulation ability and a sporulation procedure which involves transferring cells during logarithmic growth from acetate vegetative medium to a nitrogen-free sporulation medium. Premeiotic DNA synthesis, observed during sporulation between 4 and 8 h, differs from synthesis during vegetative growth in that the former utilizes internal pool precursors, while the latter does not show a comparable recycling of radioactive label from purines and pyrimidines. A sporulation deficient mutant that exhibits no premeiotic DNA synthesis suggests that premeiotic and vegetative DNA syntheses are under separate genetic controls. Two new developmental stages are recognized in sporulating cells. The first, referred to as readiness, is a stage in which the cells are able to sporulate in water without further stimulus by the sporulation medium, but will revert to mitotic growth upon transfer back to vegetative medium. This is a prolonged stage, beginning at 3 h, before premeiotic DNA synthesis starts and ending at 7 h, after the first cells finish their synthetic period. The developmental stage that follows is characterized by the inability of cells either to grow vegetatively or to sporulate, when transferred back to vegetative medium. This is a short-lived stage, detectable around 7 h in sporulation medium, that is, approximately at the time of meiotic prophase. Full commitment, beginning at 7-8 h, comes next and is evident by the normal sporulation of cells upon transfer to vegetative medium.