RNA Polymerase II evolution and adaptations: Insights from Plasmodium and other parasitic protists.

The C-terminal domain (CTD) of RNA polymerase II plays a crucial role in regulating transcription dynamics in eukaryotes. The phosphorylation of serine residues within the CTD controls transcription initiation, elongation, and termination. While the CTD is highly conserved across eukaryotes, lower eukaryotes like protists, including Plasmodium, exhibit some differences. In this study, we performed a comparative analysis of CTD in eukaryotic systems to understand why the parasites evolved in this particular manner. The Plasmodium falciparum RPB1 is exceptionally large and feature a gap between the first and second heptad repeats, resulting in fifteen canonical heptad repeats excluding the initial repeat. Analysis of this intervening sequence revealed sub motifs of heptads where two serine residues occupy the first and fourth positions (SXXS). These motifs lie in the intrinsically disordered region of RPB1, a characteristic feature of the CTD. Interestingly, the SXXS sub-motif was also observed in early-divergingeukaryotes like Leishmania major, which lack canonical heptad repeats. Furthermore, eukaryotes across the phylogenetic tree revealed a sigmoid pattern of increasing serine frequency in the CTD, indicating that serine enrichment is a significant step in the evolution of heptad-rich RPB1. Based on these observations and analysis, we proposed an evolutionary model for RNA Polymerase II CTD, encompassing organisms previously deemed exceptions, notably Plasmodium species. Thus, our study provides novel insights into the evolution of the CTD and will prompt further investigations into the differences exhibited by Plasmodium RNA Pol II and determine if they confer a survival advantage to the parasite.