ABSTRACT
In mode 1, ‘minimalist* enzymes comprise essentially only the catalytic domain (Fig. 1A,B).
They may recognize large, well-structured RNAs, as exemplified by the recently characterized 23S rRNA m3W1915 methyltransferase RlmH (formerly YbeA) from the SPOUT superfamily that acts on a fully assembled ribosome49 and, according to a docking model, forms extensive contacts with the interfaces of both the 30S and SOS subunit.50 Another example is provided by the cap-specific RNA 2'-0 -methyltransferase VP39,51 which binds conformationally flexible mRNA and therefore must recognize very specific structural features. The enzymes utilizing mode 1 for binding often increase the size of the surface available to interact with the macromolecular RNA substrate by forming homooligomers comprising several identical copies of the catalytic domain (as in the case of RlmH) or by developing ‘decorations* in the form of various protrusions that are used to recognize particular features of the substrate (as in the case of cap-recognition by VP39).
