Single-stranded replication intermediates of ribosomal DNA replicons of pea

Replication of ribosomal DNA replicons in cells of Pisum sativum (cv. Alaska) occurs bidirectionally by displacement loops. Replication is initiated on opposite parental strands and nascent chains are elongated moving 5'----3' along each parental template. Replicative intermediates were analyzed by 2-dimensional agarose gel electrophoresis under neutral--neutral and neutral--alkaline conditions. Southern blots of ribosomal DNA fragments separated in the second dimension under neutral conditions show slowly migrating replicative fragments that hybridize with specific probes in a manner consistent with bidirectional replication. The replicative fragments are present in root meristems with cells in S phase; they are absent or few in number in meristems with cells in G2 phase. The following observations indicate that the replicative fragments are single stranded. The apparent length of the replicative fragments is not the same when separated under neutral and alkaline conditions. They contain rDNA without breaks and they do not exhibit the smaller nascent chains expected from replication bubbles and forks. They are not cleaved by restriction enzymes that require duplex DNA as substrate and they are digestible by S1 nuclease.