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P.3 left column 4th paragraph:"Unlike replication, which involves a single investment per cell division, the total cost of transcription depends on the lifespan of a cell, because transcripts are typically degraded and replaced on time scales shorter than cell-division times. Several transcription-associated costs are general across bacteria and eukaryotes.
The primary investment is the synthesis of ribonucleotides, which requires one less step than that for deoxyribonucleotide synthesis, leading to an average cost of ∼ 46 P per ribonucleotide monophosphate. Assuming that ribonucleotides are efficiently recycled (meaning that, because nuclease activity does not consume ATP, the cost associated with turnover consists of the two phosphates needed to recharge them discussed below), the total number of de novo nucleotide syntheses associated with a particular gene is a function of the average steady-state number of mRNAs (Nr) and the length of the mature mRNA (Lr,mat) (including the polyadenylation tail, which when present often has a length on the order of ∼100 nucleotides), yielding a total cost of biosynthesis of 46 ·NrLr,mat P." Supplementary Materials p.3 left column 2nd paragraph:"Ribonucleotide synthesis stops one step short of that for deoxyribonucleotide synthesis, requiring two fewer ATPs per base. Following the points made above, [investigators] will assume an average cost of 46 P per ribonucleotide in its monophosphate form." |