Diversity in isochore structure among cold-blooded vertebrates based on GC content of coding and non-coding sequences

To review the general consideration about the different compositional structure of warm and cold-blooded vertebrates genomes, we used of the increasing number of genetic sequences, including coding (exons) and non-coding (introns) regions, that have been deposited on the databases throughout last years. The nucleotide distributions of the third codon positions (GC3) have been analyzed in 1510 coding sequences (CDS) of fish, 1414 CDS of amphibians and 320 CDS of reptiles. Also, the relationship between GC content of 74, 56 and 25 CDS of fish, amphibians and reptiles, respectively and that of their corresponding introns (GCI) have been considerated. In accordance with recent data, sequence analysis showed the presence of very GC3-rich CDS in these poikilotherm vertebrates. However, very high diversity in compositional patterns among different orders of fish, amphibians and reptiles was found. Significant positive correlations between GC3 and GCI was also confirmed for the genes analyzed. Nevertheless, introns resulted to be poorer in GC than their corresponding CDS, this difference being larger than in human genome. Because the limited number of available sequences including exons and introns we must be cautious about the results derived from them. However, the indicious of higher GC richness of coding sequences than of their corresponding introns could aid to understand the discrepancy of sequence analysis with the ultracentrifugation studies in cold-blooded vertebrates that did not predict the existence of GC-rich isochores.