THE RIBOSOMAL-RNA-ENCODING DNA ARRAY HAS AN ALTERED STRUCTURE IN TOPOISOMERASE-I MUTANTS OF SACCHAROMYCES-CEREVISIAE

All the chromosomes from isogenic TOP1 and top1 strains have similar m obility on pulsed-field gels except for chromosome XII, which fails to migrate into the gels in top1 mutants. Chromosome XII contains the ta ndem repeats of rRNA-encoding DNA (rDNA). When a segment of chromosome XII containing only rDNA is transferred to chromosome III by a recomb ination event, chromosome III fails to enter a pulsed-field gel in ext racts from top1 strains, indicating that the aberrant migration of chr omosome XII in top1 mutants is caused by the presence of rDNA. Failure of chromosome XII to migrate into a pulsed-field gel occurs only in p reparations from exponentially growing top1 cultures and not in prepar ations from stationary-phase top1 cultures. rDNA from a top1 strain do es enter the gel if it is cut with an enzyme (Pst I) that cuts the tan dem rDNA array into single 9-kb repeat units, indicating that more tha n a single repeat unit is required to maintain the aberrant structure.