Other RNA Processing Reactions
The precursor tRNAs in yeast contain an intervening sequence. This is removed in a more traditional set of enzymatic cleavage and ligation reactions (Fig. 5.23).
Plant viruses frequently have RNA genomes. These viruses can them-selves have viruses. These are known as virusoids, and they can grow in cells only in the presence of a parental virus. Virusoids do not encode any proteins, but they are replicated. Part of their replication cycle requires the specific cleavage of their RNA molecules. This they do in a self–cutting reaction. Symons and Uhlenbeck have investigated the
minimal nucleotide requirements for self–cleavage of these molecules. Remarkably, it is a scant 25 nucleotides that can form into a functional hammer-headed structure. Another general structure for self-cutting RNA molecules is a simple hairpin.
Figure 5.23 The pathwayfor cutting and splicing tRNA followed by the yeast Sac-charomyces.
Figure 5.24 Editing of pre-mRNA by a guide RNA using transesterifications.
Simple editing of a nucleotide or two has been observed in a mammalian RNA, but in the mitochondrion of some protozoa far more extensive editing has been observed. This raises the question of just where the informa-tion for the editing is stored. Changing a single base can conceivably be a result of a set of reactions catalyzed by enzymes designed for just the sequence at which the change occurs, but in the more dramatic exam-ples of editing, in which more than 50 U’s are inserted to produce the final edited sequences, far too many different enzymes would be re-quired. Initial examinations of the DNA of the organisms, both with computer searches of known sequence and hybridization studies, failed to reveal any sequences that could have encoded the edited sequence. Eventually it was found that the information for the edited sequences was carried in short RNAs complementary to segments of the final edited sequence. These are called guide sequences. Although cutting and re-ligation could be the pathway for editing, intermediates in editing are found that indicate instead, that the guide sequences transfer U’s from their 3’ ends to the necessary positions in the mRNA by transesterifica-tion reactions analogous to those used in splicing (Fig. 5.24).
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