REPLICATION OF A COMMON FRAGILE SITE, FRA3B, OCCURS LATE IN S-PHASE AND IS DELAYED FURTHER UPON INDUCTION - IMPLICATIONS FOR THE MECHANISM OF FRAGILE SITE INDUCTION

The FRA3B at 3p14.2 is the most highly expressed of the common fragile sites observed when DNA replication is perturbed by aphidicolin or fo late stress, The molecular basis for chromosome fragility at FRA3B is unknown. In contrast to the rare fragile sites, including FRAXA, no re peat motifs, such as trinucleotide repeats, have been identified withi n FRA3B, Several lines of evidence suggest that fragile sites are regi ons of DNA whose replication is unusually sensitive to interference, W e have used fluorescence in situ hybridization to determine the relati ve timing of replication of FRA3B sequences, Our studies revealed that FRA3B sequences are late replicating. Exposure to aphidicolin, an inh ibitor of both DNA polymerase alpha and delta, results in a reproducib le delay in the timing of replication, and some cells enter G(2) witho ut having completed replication of FRA3B sequences, Our results suppor t a model in which common fragile sites are sequences that initiate re plication late in S phase or are slow to replicate, and the chromosoma l breaks and gaps observed in metaphase cells are due to unreplicated DNA.