Synthesis of two DNA-dependent RNA polymerases in yeast

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Specific activities of Saccharomyces cerevisiae RNA polymerases I and II were measured in cells growing under different nutrient conditions and throughout the mitotic cell cycle. The specific activity of RNA polymerase I (possibly the ribosomal polymerase) does not vary during the yeast cell cycle. In contrast the specific activity of RNA polymerase II (messenger polymerase) increases during the first third of the cycle and thereafter declines. The independent regulation of synthesis of these two enzymes is further emphasised by observations on the response to different nutrient conditions. Shifting cells from minimal to rich medium led to enhanced RNA polymerase I activity but very little change in activity of RNA polymerase II. Furthermore the activity of RNA polymerase I varies directly with change in growth rate whereas the activity of RNA polymerase II is approximately constant over a range of growth rates. From this data it is suggested: ( i) The synthesis of these two enzymes is independently regulated; ( ii) RNA polymerase I is synthesised continuously throughout the cycle whereas RNA polymerase II is synthesised periodically early in the cell cycle.