Until about 20 years ago, all known enzymes were proteins. But then it was discovered that some RNA molecules can act as enzymes; that is, catalyze covalent changes in the structure of substrates (most of which are also RNA molecules). Catalytic RNA molecules are called ribozymes.
Most classes of RNAAll living things synthesize an enzyme - called Ribonuclease P - that cleaves the head (5') end of the precursors of transfer RNA (tRNA) molecules.
In bacteria, ribonuclease P is a heterodimer containingThe splicing reaction is self-contained; that is, the intron - with the help of associated proteins - splices itself out of the precursor RNA.
Once excision of the intron and splicing of the adjacent exons are completed, the story is over. In other words, although the action is catalyzed by the RNA, only a single molecule of substrate is involved (unlike protein enzymes that repeatedly catalyze a reaction).
However, synthetic versions of Group I introns made in the laboratory can - in vitro - act repeatedly; that is, like true enzymes.The DNA of some Group I introns includes an open reading frame (ORF) that encodes a transposase-like protein that can make a copy of the intron and insert it elsewhere in the genome.
| Link to a discussion of transposons. |
All the Group I introns share a characteristic secondary structure and mode of action that distinguishes them from the next group.
Because their secondary structure and the details of the splicing reaction differ from the rRNA introns discussed above, these are called Group II introns.
The DNA of some Group II introns also includes an open reading frame (ORF) that encodes a transposase-like protein that can make a copy of the intron and insert it elsewhere in the genome.
The spliceosome that removes introns and splices the exons of most nuclear genes is composed of 5 kinds of small nuclear RNA (snRNA) molecules and a large number of protein molecules. It is the snRNA - not the protein - that catalyzes the splicing reactions.
The molecular details of the reactions are similar to those of Group II introns, and this has led to speculation that this splicing machinery evolved from them.
Both viroids and virusoids are responsible for a number of serious diseases of economically important plants; e.g. the coconut palm and chrysanthemums. (The problem is so severe with chrysanthemums that all growers in the U.S. now secure their stock from a few companies that raise the plants in "clean" rooms using stringent precautions to prevent infection by the viroid.)
But if RNA can serve both as a
Yes, the ribosome turns out to be a ribozyme.
Ribosomes are huge aggregates containing 3 (4 in eukaryotes) rRNA molecules and scores of protein molecules.
The three-dimensional structure of the large (50S) subunit of a bacterial ribosome was published in August 2000. It clearly shows that formation of the peptide bond that links each amino acid to the growing polypeptide chain is catalyzed by the 23S RNA molecule in the large subunit. The 31 proteins in the subunit probably provide the scaffolding needed to maintain the tertiary structure of the RNA.
| Link to discussion of ribosome structure and function. |
In today's world, RNA polymerases - made of protein - make the RNA molecules (using the antisense strand of DNA as a template [View]). Could RNA alone have done it?
It can be done in the laboratory. Johnston et al. report in Science, 292:1319, 18 May 2001, their creation of a synthetic RNA molecule that when presented with single-stranded RNA templates, polymerizes ribonucleotide triphosphates into strands of RNA complementary to the template. Their synthetic ribozyme was able to faithfully incorporate up to 14 nucleotides into the complementary strand of RNA.
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