Nuclear pre-mRNA introns: analysis and comparison of intron sequences fromTetrahymena thermophilaand other eukaryotes

Abstract

We have sequenced 14 introns from the ciliate Tetrahymena thermophila and include these in an analysis of the 27 intron sequences available from seven T. thermophila protein-encoding genes. Consensus 5' and 3' splice junctions were determined and found to resemble the junctions of other nuclear pre-mRNA introns. Unique features are noted and discussed. Overall the introns have a mean A + T content of 85% (21% higher than neighbouring exons) with smaller introns tending towards a higher A + T content. Approximately half of the introns are less than 100 bp. Introns from other organisms (approximately 30 of each) were also examined. The introns of Dictyostelium discoideum, Caenorhabditis elegans and Drosophila melanogaster, like those of T. thermophila, have a much higher mean A + T content than their neighbouring exons (greater than 20%). Introns from plants, Neurospora crassa and Schizosaccharomyces pombe also have a significantly higher A + T content (10%-20%). Since a high A + T content is required for intron splicing in plants (58), the elevated A + T content in the introns of these other organisms may also be functionally significant. The introns of yeast (Saccharomyces cerevisiae) and mammals (humans) appear to lack this trait and thus in some aspects may be atypical. The polypyrimidine tract, so distinctive of vertebrate introns, is not a trait of the introns in the non-vertebrate organisms examined in this study.

Expression of the gene encoded by a family of macronuclear chromosomes generated by alternative DNA processing inOxytricha fallax

“…The extended consensus sequences and high A+ T composition are similar to those found in the introns of other eukaryotes, including Tetrahymena (20), and to that of the only other known hypotrich intron (7). A possible branch point sequence YTRAT, consistent with the mammalian branch site consensus YTRAY (21), is seen in all introns except vA and vB of iI, which have the consensus YCRAT (Fig.…”

Osmoregulation of a Pyrroline-5-Carboxylate Reductase Gene in Arabidopsis thaliana

“…A T -P5C1 contains six short introns (81,93, 125, 115, 78, aind 106 bp). They a11 contain the GT and AG dinucleotide consensus at their respective 5' and 3' ends (Csank et al, 1990).…”

Genomic organization of three novel toxins from the scorpion Buthus martensi Karsch that are active on potassium channels

“…(2) All the genomic DNA sequences of the toxins consist of two exons disrupted by an intron of 80-130 bp inserted within the signal peptide region near its C-terminus. The introns agree with the GT\AG splice junctions and are rich in AjT content (more than 80 %), in contrast with their exons (less than 60 %) [26]. The intron sequences share high similarity with toxins from the same subgroup, but have no similarity with toxins from different subgroups.…”