SPECIFIC COMPOSITIONAL PATTERNS OF SYNONYMOUS POSITIONS IN HOMOLOGOUSMAMMALIAN GENES

All 69 homologous coding sequences that are currently available in fou r mammalian orders were aligned and the synonymous (ie., third) positi ons of quartet (fourfold degenerate) codons were divided into three cl asses (that will be called conserved, intermediate, and variable), acc ording to whether they show no change, one change, and more than one c hange, respectively. The three classes were analyzed in their composit ional patterns. In the majority of GC-rich genes, the three classes of positions (but especially conserved positions) exhibited significantl y different base compositions compared to expectations based on a ''ra ndom'' substitution process from the ''ancestral'' (consensus) sequenc e to the present-day (actual) sequences. Significant differences were rare in GC-poor genes. An analysis of the present results indicates th at natural selection plays a role in the synonymous nucleotide substit ution process, especially in GC-rich genes which represent the vast ma jority of mammalian genes.