There is a steady-state transcriptome in exponentially growing yeast cells.

The growth of yeast cells in batches in glucose-based media is a standard condition in most yeast laboratories. Most gene expression experiments are done by taking this condition as a reference. Presumably, cells are in a stable physiological condition that can be easily reproduced in other laboratories. With this assumption, however, it is necessary to consider that the average amount of the mRNAs per cell for most genes does not change during exponential growth. That is to say, there is a steady-state condition for the transcriptome. However, this has not been rigorously demonstrated to date. In this work we take several cell samples during the exponential phase growth to perform a kinetic study using the genomic run-on (GRO) technique, which allows simultaneous measurement of the amount of mRNA and transcription rate variation at the genomic level. We show here that the steady-state condition is fulfilled for almost all the genes during most exponential growth in yeast extract-peptone-dextrose medium (YPD) and, therefore, that simultaneous measures of the transcription rates and the amounts of mRNA can be used for indirect mRNA stability calculations. With this kinetic approach, we were also able to determine the relative influence of the transcription rate and the mRNA stability changes for the mRNA variation for those genes that deviate from the steady state.