Ohman-Hedén M, Ahgren-Stålhandske A, Hahne S, Sjöberg B M
Department of Molecular Biology, University of Stockholm, Sweden.
Mol Microbiol. 1993 Mar;7(6):975-82. doi: 10.1111/j.1365-2958.1993.tb01189.x.
The bacteriophage T4 nrdB gene, encoding the ribonucleotide reductase small subunit, contains a self-splicing group IA2 intron with an ochre codon in frame with the preceding exon sequence. The stop codon was changed to an amino acid codon and splicing efficiency was compared with that of the wild type in the presence and absence of translation. In vivo the mutant has a much lower efficiency for producing a mature transcript than the wild type. Also, the relative production of the full-length translation product is correspondingly lower in the mutant than in the wild type. These results confirm the importance of the stop codon, which spans the splice site of the nrdB intron. The occurrence of stop codons in 56 group I introns in protein-encoding genes was investigated. In 33 of those translation is terminated upstream of the first common elements of the catalytic core, of group I introns. In the rest translation is terminated in intron regions outside the heart of the catalytic core, with one exception. Our observations suggest that in situations where transcription and translation are coupled events there has been an evolutionary pressure to preserve stop codons in the 5'-region of these introns or to prevent translational termination from occurring in vital parts of the introns.
噬菌体T4的nrdB基因编码核糖核苷酸还原酶小亚基，它含有一个自我剪接的IA2类内含子，该内含子中的一个赭石密码子与前一个外显子序列在同一阅读框内。将该终止密码子替换为一个氨基酸密码子，并在有无翻译的情况下，将剪接效率与野生型进行比较。在体内，与野生型相比，突变体产生成熟转录本的效率要低得多。此外，突变体中全长翻译产物的相对产量相应地低于野生型。这些结果证实了跨越nrdB内含子剪接位点的终止密码子的重要性。研究了蛋白质编码基因中56个I类内含子中终止密码子的出现情况。其中33个内含子的翻译在I类内含子催化核心的首个共同元件上游终止。其余的内含子翻译在催化核心中心之外的内含子区域终止，只有一个例外。我们的观察结果表明，在转录和翻译偶联的情况下，存在一种进化压力，促使这些内含子5'区域保留终止密码子，或防止在内含子的关键部分发生翻译终止。
