Replication of yeast rDNA initiates downstream of transcriptionally activegenes

In the yeast S. cerevisiae, ARS (autonomously replicating sequence) element s located in the intergenic spacers of the rRNA gene locus are infrequently activated as origins of replication. We analyzed the rARS activation with a combination of neutral/neutral (N/N) two-dimensional (2D) gel electrophor esis and either the intercalating drug psoralen, which in vivo specifically marks the transcribing gene copies, or the selective accessibility of rest riction sites in transcriptionally active genes. We found that initiation o f replication starts at those rARSs placed immediately downstream of transc ribing rRNA genes. This correlation between transcription and replication i s consistent with the presence of nucleosome-free enhancers at each transcr iptionally active gene copy and suggests that the transcription factor Abf1 p is involved in replication initiation at the ARS in the rDNA gene locus.