INTRON-DEPENDENT ENZYMATIC FORMATION OF MODIFIED NUCLEOSIDES IN EUKARYOTIC TRANSFER-RNAS - A REVIEW

In eukaryotic cells, especially in yeast, several genes encoding tRNAs contain introns. These are removed from pre-tRNAs during the maturati on process by a tRNA-specific splicing machinery that is located withi n the nucleus at the nuclear envelope. Before and after the intron rem oval, several nucleoside modifications are added in a stepwise manner, but most of them are introduced prior to intron removal. Some of thes e early nucleoside modifications are catalyzed by intron-dependent enz ymes while most of the others are catalyzed in an intron-independent m anner. In the present paper, we review all known cases where the nucle oside modifications were shown to depend strictly on the presence of a n intron. These are pseudouridines at anticodon positions 34, 35 and 3 6 and 5-methylcytosine at position 34 of several eukaryotic tRNAs. One common property of the corresponding intron-dependent modifying enzym es is that their activities are essentially dependent on the local spe cific architecture of the pre-tRNA molecule that comprises the anticod on stem and loop prolonged by the intron domain. Thus introns clearly serve as internal (cis-type) RNAs that guide nucleoside modifications by providing transient target sites in tRNA for selected nuclear modif ying enzymes. This situation may be similar to the recently discovered (trans-type) snoRNA-guided process of ribose methylations of ribosoma l RNAs within the nucleolus of eukaryotic cells.