Abstract
5-Methylcytosine (5mC) levels were determined in compositional DNA fractions corresponding to different isochore families from the genomes of
Xenopus, chicken, mouse and human, four vertebrates which show different isochore patterns. The results obtained indicate that: (i) positive correlations exist between the 5mC levels and the GC levels of isochores within any given genome; and (ii) DNA from
Xenopus isochore families is twice as methylated as DNA from the isochores having the same GC levels from mouse, human and chicken. Moreover, the positive correlations holding between CpG levels and the GC
3 levels of coding sequences of warm-blooded vertebrates were shown to comprise two regions with a border at approx. 75% GC
3. The correlation corresponding to the higher region (which comprises only very rare high GC
3 values in the case of
Xenopus) has a higher slope than that corresponding to the lower GC
3 values, a phenomenon due in all likelihood, to the increasing contribution of CpG islands. Finally, the observed/expected CpG ratio is higher in
Xenopus than in warm-blooded vertebrates.