MOLECULAR ANALYSIS OF PHASEOLUS-VULGARIS RDNA UNIT AND CHARACTERIZATION OF A SATELLITE DNA HOMOLOGOUS TO IGS SUBREPEATS

A detailed molecular analysis of the Phaseolus vulgaris L. rDNA unit w as carried out. The entire internal transcribed spacer (ITS), includin g 5.8S RNA and both 18S and 26S flanking regions, was sequenced. Other fragments of particular interest were analyzed: the putative polymera se I promoter, including relevant upstream structural elements, and se veral copies of the intergenic spacer subrepeated elements. Five thous and copies of rDNA units and 100,000 copies of IGS subrepeats (IGS-s.r .) were found per haploid genome. Sizes of bean rDNA units were found highly homogeneous with a major class of 12 kb and a minor class of 15 kb. Size variants were shown to be associated with a variable number of the IGS tandem subrepeats. A 166 bp repeated element (CC4) was foun d to share extensive homologies with the IGS subrepeated element. A de tailed study of this element showed that 4,000 copies were arranged in tandemly repeated structures. Comparison of CC4 and IGS subrepeats in the bean genome, using specific oligonucleotides, showed CC4 to be in dependent of rDNA genes.