Non–coding RNA genes and the modern RNA world

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Abstract
Although some textbooks still discuss only three types of RNA (ribosomal RNA, transfer RNA and messenger RNA), many other non-coding RNA (ncRNA) species have been isolated anecdotally. Systematic screens looking for more ncRNA genes have been undertaken recently. Two large families of small nucleolar RNAs are involved in directing site-specific modifications of rRNAs and other RNAs, and the number of known genes in these families continues to grow rapidly. Three recent papers describe a new large eukaryotic RNA gene family, the microRNAs — tiny 21–24-nucleotide RNAs that are probably acting as translational regulators of protein-coding mRNAs. Four recent papers describe screens for new ncRNA genes in Escherichia coli, leading to the experimental confirmation of more than 30 new non-coding transcripts of as yet unknown function, and the computational prediction of many more. RNA genes have been thought of as rare relics of a primordial “RNA world” that has largely been replaced by more efficient proteins. Now, though, it seems that ncRNAs might be numerous and highly adapted in their roles in modern organisms. RNA is particularly well suited to the job of specific recognition of other RNAs by complementary base pairing. Evolution might favour ncRNAs instead of proteins in certain roles; for instance, as post-transcriptional regulatory molecules that interact with specific mRNAs.