Many expressed genes in bacteria and yeast are transcribed only once per cell cycle.

The steady-state levels of all mature transcripts expressed in bacteria and yeast have been cataloged, but we do not yet know the numbers of nascent transcripts and so RNA polymerases engaged on all genes. Such catalogs are presented here. As mRNA levels depend on the balance between synthesis and degradation, we use published data to calculate the numbers of engaged polymerases required to maintain these levels in the face of the known rate of degradation. Most genes, including essential ones, prove not to be transcribed most of the time, and many produce only one message per cell cycle. Some cells even fail to produce an essential message during a cycle, and so must depend on their mother's messages and/or proteins for survival. We speculate that evolution sets the rate of message production so low to conserve energy, minimize transcription-induced mutation, and permit regulation over the widest range.