REPLICATION FORK BARRIERS IN THE XENOPUS RDNA

To investigate replication fork progression along the tandemly repeate d rRNA genes of Xenopus laevis and Xenopus borealis, rDNA replication intermediates from dividing tissue culture cells were analyzed by two- dimensional gel electrophoresis. Analysis of the direction of replicat ion in the rRNA coding regions revealed replication forks moving in bo th directions. However, in both frog species, polar replication fork b arriers (RFB) arresting forks approaching the rRNA transcription units from downstream were identified. Whereas in X.borealis the RFB maps t o a defined site close to the transcription terminator, in X.laevis th e arrest of fork movement can occur at multiple positions throughout a 3' flanking repetitive spacer region. A short DNA element located nea r the respective RFB sites is shared between these two related frog sp ecies, suggesting its possible involvement in the arrest of replicatio n fork movement. In a subset of rDNA repeats, these barriers cause an absolute block to replication fork progression, defining the sites whe re replicon fusion occurs, whereas in the remainder repeats, most prob ably in the non-transcribed gene copies, the replication machinery can pass the RFB sequences and replicate the rRNA transcription unit in a 3'-to-5' direction.