ROLES OF NOVOBIOCIN-SENSITIVE TOPOISOMERASES IN CHLOROPLAST DNA-REPLICATION IN CHLAMYDOMONAS-REINHARDTII

We have examined DNA replication in Chlamydomonas reinhardtii chloropl asts in vivo when chloroplast type II topoisomerases are inactivated w ith sublethal doses of novobiocin. DNA replication is at first inhibit ed under these conditions. However, after a delay of several hours, ch loroplast chromosomes initiate a novobiocin-insensitive mode of DNA re plication. This replication starts preferentially near a hotspot of re combination in the large inverted repeats, instead of from the normal chloroplast origins, oriA and oriB. It replicates one, but not the oth er single-copy region of the chloroplast chromosome. We speculate that novobiocin-insensitive DNA replication in chloroplasts requires recom bination in this preferred initiation region.