METHYLATION PATTERNS IN THE ISOCHORES OF VERTEBRATE GENOMES

5-Methylcytosine (5mC) levels were determined in compositional DNA fra ctions corresponding to different isochore families from the genomes o f Xenopus, chicken, mouse and human, four vertebrates which show diffe rent isochore patterns. The results obtained indicate that: (i) positi ve correlations exist between the 5mC levels and the GC levels of isoc hores within any given genome; and (ii) DNA from Xenopus isochore fami lies is twice as methylated as DNA from the isochores having the same GC levels from mouse, human and chicken. Moreover, the positive correl ations holding between CpG levels and the GC(3) levels of coding seque nces of warm-blooded vertebrates were shown to comprise two regions wi th a border at approx. 75% GC(3). The correlation corresponding to the higher region (which comprises only very rare high GC(3) values in th e case of Xenopus) has a higher slope than that corresponding to the l ower GC(3) values, a phenomenon due in all likelihood, to the increasi ng contribution of CpG islands. Finally, the observed/expected CpG rat io is higher in Xenopus than in warm-blooded vertebrates. (C) 1997 Els evier Science B.V.