Comparative RNomics and modomics in mollicutes: Prediction of gene function and evolutionary implications

Stable RNAs are central to protein synthesis. Ribosomal RNAs make the core of the ribosome and provide the scaffold for accurate translation of mRNAs by a set of tRNA molecules each carrying an activated amino acid. To fulfill these important cellular functions, both rRNA and tRNA molecules require more than the four canonical bases and have recruited enzymes that introduce numerous modi. cations on nucleosides. Mollicutes are parasitic unicellular bacteria that originated from gram-positive bacteria by considerably reducing their genome, reaching a minimal size of 480 kb in Mycoplasma genitalium. By analyzing the complete set of tRNA isoacceptors ( tRNomics) and predicting the tRNA/rRNA modi. cation enzymes ( Modomics) among all sequenced Mollicutes ( 15 in all), our goal is to predict the minimal set of RNA modi. cations needed to sustain accurate translation of the cell's genetic information. Building on the known phylogenetic relationship of the 15 Mollicutes analyzed, we demonstrate that the solutions to reducing the RNA component of the translation apparatus vary from one Mollicute to the other and often rely on co-evolution of specific tRNA isoacceptors and RNA modification enzymes. This analysis also reveals that only a few modi. cation enzymes acting on nucleotides of the anticodon loop in tRNA (the wobble position 34 as well as in position 37, 3'-adjacent to anticodon) and of the peptidyltransferase center of 23S rRNA appear to be absolutely essential and resistant to gene loss during the evolutionary process of genome reduction.