Transcription of functionally related constitutive genes is not coordinated

Expression of an individual gene can vary considerably among genetically identical cells due to stochastic fluctuations in transcription. However proteins comprising essential complexes or pathways have similar abundances and lower variability. It is not known whether coordination in the expression of subunits of essential complexes occurs at the level of transcription, mRNA abundance, or protein expression. To directly measure the level of coordination in the expression of genes, we used highly sensitive fluorescence in situ hybridization (FISH) to count individual mRNAs of functionally related and unrelated genes within single Saccharomyces cerevisiae cells. Our results revealed that transcripts of temporally induced genes are highly correlated in individual cells. But in contrast, transcription of constitutive genes encoding essential subunits of complexes is not coordinated due to stochastic fluctuations. Therefore the coordination of these functional complexes must occur post-transcriptionally, and likely post-translationally.