SEGMENTED GENE CONVERSION AS A MECHANISM OF CORRECTION OF 18S RIBOSOMAL-RNA PSEUDOGENE LOCATED OUTSIDE OF RDNA CLUSTER IN DROSOPHILA-MELANOGASTER

The peculiarities of the sequences of 18S rDNA included in a 90-kb DNA segment cloned in YAC vector are described. This heterochromatic segm ent is situated on the X chromosome distal to the main rDNA cluster. T he pseudo 18S rDNA sequence comprised undamaged stretches of rDNA inte rspersed with segments characterized by high density of nucleotide sub stitutions and insertions/deletions. The observed patchwork arrangemen t of unaltered rDNA sequences was considered as evidence of segmented gene conversion events between the normal and damaged genes which are thought to constitute one of the mechanisms of rDNA array homogenizati on. The 18S rDNA fragment (510 bp) located nearby, homologous to the i nternal, undamaged part of pseudo 18S rDNA, carries comparable density of randomly distributed nucleotide substitutions with no evidence of correction.