[Distribution of 5-methylcytosine in pyrimidine oligonucleotides of higher plant DNA].

The level of pyrimidine clusters (isopliths) and distribution of 5-methylcytosine in DNAs of some archegonial (ferns, ginkgo) and flowering (cordyline, tulip, wheat) plants has been studied. DNA from Cordyline australis (Liliaceae) is one of the GC-type (GC = 57.5 mol.%), has very low methylation level (amount of m5C is about 1 mol.%) and significantly differs from other species DNAs studied in terms of pyrimidine distribution patterns, i.e. amount of dipyrimidine fragments is higher than that of monopyrimidine ones. All other plant DNAs under study are of the AT-type, have similar pyrimidine distribution patterns and are characterized by high degree of pyrimidine blocking; specific differences in the level (approximately 10-fold) and type of genome methylation are found. In all DNAs studied 5-methylcytosine is unevenly distributed in pyrimidine isopliths: the degree of cytosine methylation [m5C(C+m5C)] is decreased with an increase in the length of isopliths, irrespective of the ratio of thymine and cytosine residues. 5-methylcytosine is mainly accumulated in mono- and dipyrimidine fragments (60--68%). Specific differences in 5-methylcytosine content in trinucleotides and long-chain pyrimidine oligonucleotides are revealed.