processome

processome

In molecular biology, a processome is a large ribonucleoprotein complex that participates in the early stages of ribosomal RNA (rRNA) maturation. The term is most commonly used to describe the 90S pre-ribosomal particle found in eukaryotic cells, which assembles in the nucleolus and gives rise to the 40S and 60S ribosomal subunits. The processome contains hundreds of proteins, many of which are ribosome biogenesis factors, along with the 35S pre-rRNA transcript in yeast (18S, 5.8S, and 25S rRNAs) or the 45S pre-rRNA in mammals (18S, 5.8S, and 28S rRNAs).

Assembly of the processome proceeds in a stepwise manner, beginning with the binding of small nucleolar ribonucleoproteins (snoRNPs) that guide chemical modifications of the pre-rRNA. Subsequent recruitment of the U3 snoRNP, a specific small nucleolar RNA essential for early cleavages, drives the formation of the central domain of the 18S rRNA. Multiple ATP‑dependent remodeling complexes, such as the Rix1–Cdc73–Nipps homolog (RCN) complex and the ATPases Rix10 and Rix7, act on the core scaffold to facilitate the processing reactions and the release of specific maturation factors.

The processome is responsible for cleaving the primary pre‑rRNA transcript at several precise sites, trimming the external and internal transcribed spacers, and directing the join of rRNA secondary structures. Defects in processome assembly or function lead to nucleolar stress and, in humans, can cause disorders such as ribosomopathies. The study of the processome has also provided insights into the coordination of transcription and ribosome synthesis, illustrating how RNA polymerase II and ribosomal biogenesis are tightly coupled within the nucleolus.