Statistical analysis of complete bacterial genomes: Avoidance of palindromes and restriction-modification systems

Avoidance of 4-, 5-, and 6-letter palindromes is observed in many prokaryot ic genomes. A large fraction of such palindromes is formed by restriction s ites of the species itself or a closely related species. One possible reaso n for that is the horizontal transfer of genes encoding restriction-modific ation systems. In organisms isolated from the action of such systems (e.g., in Mycoplasma), palindromes are not avoided. The general tendencies in pre ferences and avoidance of palindromes were studied for 33 available prokary otic genomes. The results obtained provide additional insight into the rela tionships within and between taxonomic groups.