Biocatalysis
is dominated by protein enzymes, and only a few classes
of ribozymes are known to contribute to the task of promoting biochemical
transformations. The RNA World theory encompasses the notion that
earlier forms of life made use of a much greater diversity of ribozymes
and other functional RNAs to guide complex metabolic states long before
proteins had emerged in evolution. In recent years, the discoveries
of various classes of ribozymes, riboswitches, and other noncoding
RNAs in bacteria have provided additional support for the hypothesis
that RNA molecules indeed have the catalytic competence to promote
diverse chemical reactions without the aid of protein enzymes. Herein,
some of the most striking observations made from examinations of natural
riboswitches that bind small ligands are highlighted and used as a
basis to imagine the characteristics and functions of long-extinct
ribozymes from the RNA World.