Perspective: divergent mRNA transcription machinery in Paramecium

Proper regulation of transcription involves not only quantitative control of RNA dosage but also ensuring the correct biochemical properties of transcripts. In all eukaryotes, the epigenetic landscape and the dynamic composition of the RNA Polymerase II complex (PolII) interact to control the transcription of translatable mRNA. Decades of research have described dogmatic rules for model organisms, such as the distribution of individual chromatin marks along the transcription unit or the hierarchical phosphorylation pattern in the C-terminal domain (CTD) of the largest PolII subunit RPB1. Besides this canonical mRNA transcription, there are exceptions; on the one hand, not all genes in a species follow the dogma, and on the other hand, there are species that show general divergence from the models, both in the epigenomic landscape and in the genetically encoded PolII. In the recent literature, protists in particular have shifted their attention as they show considerable differences in chromatin structure and PolII complex composition. Here, we aim to enlighten the transcription machinery of the unicellular ciliate Paramecium as an exciting model to study a divergent transcriptional machinery for vegetative mRNA and developmental ncRNA transcription.