MOLECULAR CLONING OF A PLANT rDNA FRAGMENT CONTAINING THE 5́ -PORTION OF 18S rRNA GENE AND ITS USE AS A PROBE IN INDIRECT END LABELING EXPERIMENTS

Plasmid pBG 35 contains the whole rDNA unit from flax (Linum usitatissimum L.) (Goldsbrough and Cullis, 1981). We cloned four different Eco RI restriction rDNA fragments of pBG 35 in the plasmid vector pBluescript SK+ (pBS). One of the subclones (p2.1) contained the most part of the 5 ́-end of 18S rRNA gene (1581 bp) and was used as a heterologous DNA probe in molecular hybridization experiments. By means of the “indirect end labeling” technique, the plasmid p2.1 was used to study the methylation pattern of the intergenic spacer (IGS) of rRNA genes in excised cotyledons of Cucurbita pepo L. (zucchini) after treatment with the cytokinin 6-benzyladenine (BA). Heavy methylation of cytosine residues in the sequence -CCGGboth in IGS and 18S rRNA gene were observed. The methylation pattern was not changed upon hormonal treatment with BA.