Compositional constraints and genome evolution.

Nucleotide sequences of all genomes are subject to compositional constraints that affect, to about the same extent, both coding and noncoding sequences; influence not only the structure and function of the genome, but also those of transcripts and proteins; are the result of environmental pressures; and largely control the fixation of mutations. These findings indicate that noncoding sequences are associated with biological functions; that the organismal phenotype comprises two components, the classical phenotype, corresponding to the "gene products," and a "genome phenotype," which is defined by the compositional constraints; and that natural selection plays a more important role in genome evolution than do random events.