RESUMEN
We conducted comprehensive analyses on intron positions in the Mus musculus genome by comparing genomic sequences in the GenBank database and cDNA sequences in the mouse cDNA library recently developed by Riken Genomic Sciences Center. Our results confirm that introns have a tendency to be located toward the 5' end of the gene. The same type of analysis was conducted in the coding region of seven eukaryotes (Saccharomyces cerevisiae, Plasmodium falciparum, Caenorhabditis elegans, Drosophila melanogaster, M. musculus, Homo sapiens, Arabidopsis thaliana). Introns in genes with a single intron have a locational bias toward the 5' end in all species except A. thaliana. We also measured the distance from the start codon to the position of the intron, and found that single introns prefer the location immediately after the start codon in S. cerevisiae and P. falciparum. We discuss three possible explanations for these findings: (1) they are the consequence of intron loss by reverse-transcriptase; (2) they are necessary to accommodate the function; and (3) they are concerned with the mechanism of pre-mRNA splicing.
Asunto(s)
Células Eucariotas/metabolismo , Intrones/genética , Animales , Arabidopsis/genética , Caenorhabditis elegans/genética , Codón Iniciador/genética , Codón de Terminación/genética , ADN/genética , ADN Complementario/genética , Bases de Datos de Ácidos Nucleicos , Drosophila melanogaster/genética , Exones/genética , Humanos , Plasmodium falciparum/genética , Saccharomyces cerevisiae/genética , Especificidad de la EspecieRESUMEN
The signals that direct the excision of introns from mammalian pre-mRNA are not yet well understood. However, at least three kinds of signals--5'-splice site signals, 3'-splice site signals and branch point signals--play important roles in the excision of introns. In the present paper we treat only the 5'-splice sites. In addition to a consensus sequence for 5'-splice signals, several methods have been proposed, based on a statistical model, and used to analyze relative importance of each nucleotide at each position. In our approach a nucleotide sequence is regarded as a string with symbols of 'A', 'T', 'G' and 'C'; important substrings of 5'-splice site sequences, called pattern sequences, are extracted. A pattern sequence expresses which nucleotide is needed at a limited number of positions around the 5'-splice site. It is observed that a particular pattern sequence matches predominantly 5'-splice site sequences nearest to the 5'-end of a gene and another pattern sequence matches predominantly the second nearest ones. Moreover, it is confirmed that the pattern sequences accurately predict authentic 5'-splice sites for unknown genes and explain some mutation examples.
