SOME FEATURES OF METHYLATION OF CYTOSINE RESIDUES IN RDNA OF A DIPLOID WHEAT TRITICUM-URARTU THUM. EX GANDIL. AND DIPLOID AEGILOPS-UMBELLULATA ZHUK

Methylation of cytosine residues (C methylation) in rDNA was investiga ted in two representatives of the tribe Triticeae, a diploid wheat Tri ticum urartu and diploid Aegilops umbellulata. These species are very different in the activity of the nucleolus organizer, the structure of the intergenic spacer, and the number of rRNA genes. Southern blottin g with a subsequent densitometry of autoradiograms showed that methyla tion of cytosine residues in rDNA of the studied species occurred main ly within CG dinucleotide sequences. At the same time, these cereal sp ecies were essentially different in the level and character of rDNA me thylation. Thus, the CG methylation type insignificantly exceeded the CNG type in T. urartu, whereas in Aegilops umbellulata preferential me thylation by the CG type was more expressed. Approximately 95% of the HpaII endonuclease recognition sites in rDNA of Ae, umbellulata were n ot available for restriction, whereas in T. urartu, this value was nea rly 65%. The CNG methylation type was detected using restriction endon uclease EcoRII. With this type of methylation, in approximately the si xth and third quota of all DNA repeats of Ae. umbellulata and T. urart u, respectively, only single sites of this enzyme were available for h ydrolysis. The lower extent of C methylation in T. urartu was possibly connected with the lower copy number of rDNA repeats in this species compared to Ae. umbellulata. Phenomenon of nucleolus domination in Ae. umbellulata was probably not connected with the level of C methylatio n in rDNA repeats of this species.