Cotranscriptional Moonlighting of RsmC as an RNA Chaperone Protein

Abstract

Ribosome is the ribonucleoprotein cellular machine that carries out protein biosynthesis in all kingdoms of life. During ribosome biogenesis, accurate coordination of rRNA transcription, folding, modification, processing, and assembly of r-proteins is essential to form a functional ribosome. The involvement of assembly factors is critical to synchronize various processes that co-occur with ribosome assembly. Here we report moonlighting of RsmC (ribosomal RNA small subunit methyltransferase C) as an RNA chaperone and RNA strand annealer during ribosome synthesis in cells. RsmC methylates an exocyclic amine of guanine (G) located at 1207 of helix 34 (h34) of 16S rRNA (E. coli standard nucleotide numbering). A 25-fold increase in the rate of annealing of h34 was observed in our FRET-based stopped-flow kinetic measurements. Furthermore, smFRET assays illustrated the ability of RsmC to denature the predicted hairpin structure of its substrate strand. Chaperone activity of RmsC may be critical for the formation of the 30S head domain.