Polite DNA: functional density and functional compatibility in genomes.

Certain as yet poorly defined functions of DNA appear to involve collectively domain-sized sequences. It is proposed that most sequence segments within a domain may be either functionally superfluous or instrumental, depending on how many related sequences are present in the domain. When redundant and functionally dispensable, such DNA segments presumably still have to conform to compositional or sequence-motif patterns that characterize the domain. In its relations with neighboring sequences, such DNA is required to be "polite." Polite DNA is DNA that, without being crucially involved in function, is subject to constraints of conformity and, through its base composition, respects a function for which it is not required. This concept is developed by contrasting the distribution of specific and general functions over DNA with this distribution as found in proteins and by distinguishing functional compatibility from pivotal functionality. The sequence constraints to which heterochromatin as well as, apparently, long interspersed repetitive sequences are known to be subject seem to imply that DNA, even when it does not carry out a pivotal function, is indeed, at the very least, required to be polite.