CHROMOSOMAL REPLICATION INITIATES AND TERMINATES AT RANDOM SEQUENCES BUT AT REGULAR INTERVALS IN THE RIBOSOMAL DNA OF XENOPUS EARLY EMBRYOS

We have analysed the replication of the chromosomal ribosomal DNA (rDN A) cluster in Xenopus embryos before the midblastula transition. Two-d imensional gel analysis showed that replication forks are associated w ith the nuclear matrix, as in differentiated cells, and-gave no eviden ce for single-stranded replication intermediates (RIs). Bubbles, simpl e forks and double Ys were found in each restriction fragment analysed , showing that replication initiates and terminates without detectable sequence specificity. Quantification of the results and mathematical analysis showed that the average rDNA replicon replicates in 7.5 min a nd is 9 - 12 kbp in length. This time is close to the total S phase du ration, and this replicon size is close to the maximum length of DNA w hich can be replicated from a single origin within this short S phase. We therefore infer that (i) most rDNA origins must be synchronously a ctivated soon in S phase and (ii) origins must be evenly spaced, in or der that no stretch of chromosomal DNA is left unreplicated at the end of S phase. Since origins are not specific sequences, it is suggested that this spatially and temporary concerted pattern of initiation mat ches some periodic chromatin folding, which itself need not rely on DN A sequence.