Histone H4 confers mRNA multiplexing into ribonucleoprotein particles that control the mating response in yeast

Unlike prokaryotic organisms that rely on polycistronic transcription (i.e. operons), eukaryotic messenger RNA ribonucleoprotein particles (mRNPs) are composed of multiple mRNAs and RNA-binding proteins (RBPs) (Mitchell, 2014). Eukaryote mRNPs contain monocistronic mRNAs that undergo co-translational control and can potentially encode proteins involved in the same biological process or molecular complex. Thus, mRNPs may confer combinatorial gene expression in a manner that is functionally similar to operons. Although a tempting possibility, the RNA transcripts assembly into messenger ribonucleoprotein (mRNP) particles is poorly understood. We employed transcript-specific RNA pulldowns and RNA-seq to examine the RNA content of mRNPs in yeast, and found that mRNAs encoding secreted components of the MATα mating pathway co-precipitate as a complex with specific mRNAs involved in growth. As a similar RNA complex could be identified in MATa cells, we propose that mRNAs can multiplex in trans to form discrete mRNP particles. In order to identify factors involved in RNA multiplexing, pulldowns of the mating mRNP particle transcripts, followed by mass spectrometry (RaPID-MS), were performed and revealed that a histone H4 paralog, Hhf1 {Dollard, 1994 #1617}, binds to these mRNAs and facilitates RNA association. The deletion of HHF1 alone was found to reduce assembly of the mating mRNP, without affecting mRNA levels or localization to the nER, and led to a ~50% decrease in mating. Overall, our results demonstrate that mRNAs multiplex to form functional mRNPs particle and that histone H4 plays an essential role in multiplexing and the control of cell physiology in yeast.