Replication of 5S ribosomal genes precedes the appearance of early nuclearreplication complexes

The present work shows that replication of the 5 S ribosomal genes differs in time and 3'deoxyadenosine sensitivity from replication of other nuclear genes, in Allium cepa L. root meristems. Fluorescence in situ hybridization with the pTa794 DNA probe which contains a complete 410 bp 5 S gene from T riticum aestivum allowed to detect four clusters of 5 S genes in these dipl oid cells (2n = 16), two of them in the short arm of the smallest metacentr ic chromosomal pair 7. Replication of the 5 S ribosomal genes occurred very early in interphase, as discerned by their resolution as doubled spots onl y two hours after interphase was initiated in synchronous binucleate cells. Codetection of nuclear replication (by immunodetection of 5-bromo-2'-deoxy uridine incorporation) showed that the replication of the 5 S ribosomal gen es occurred before any incorporation of 5-bromo-2'deoxyuridine could be det ected in the nuclei. The earliest Br-DNA detected in these cells followed a radial pattern from different foci apparently dispersed along some chromos omal arms. These structures seem to represent early replication complexes, as a result of the displacement of multiple DNA forks from the foci known a s pre-replication complexes where the replication machinery of the earliest replicating genes assembles. No consistent positional correlation existed between the formation of the early replication complexes and the already re plicated 5 S ribosomal clusters. Finally, nuclear replication but not that of the 5 S genes was prevented by 3'deoxyadenosine, and the earliest replic ating 5 S ribosomal gene cluster differed in both sister nuclei resulting f rom the segregation of one single chromosome in anaphase.