Three Structure-Selective Endonucleases Are Essential in the Absence of BLM Helicase in Drosophila
A. The first structure (i) represents a replication fork with a block (diamond) on the leading strand. Arrowheads on dark lines indicate the 3′ ends of the template strands; arrows on light lines indicate 3′ ends of the nascent leading (blue) and lagging (red) strands. It is possible that blocked forks can be cleaved on the lagging strand template by GEN or on the leading strand template by MUS81–MMS4. More typically, however, the fork is regressed (ii), possibly with template switching (iii). After removal of the block, DmBLM catalyzes reversal of the regressed structure to re-establish the replication fork. In the absence of DmBLM, regressed forks without or with template switching can be cut by MUS81–MMS4 or GEN, respectively (iv and v). Blocked forks can also spontaneously break (dotted line), especially if not protected by SPN-A. Collapsed forks resemble one-ended DSBs, but replication from a fork to the right converts these into DSBs (vi and vii), which are repaired by standard DSB repair pathways (see Figure S1). B. Converging replication forks (viii) sometimes experience problems that are solved through a DmBLM-dependent migration/decatenation process (ix). In the absence of DmBLM, MUS312–SLX1 cuts a fork, generating a DSB (x). It is also possible that both forks are cut, leading to DSBs on both chromatids (not shown). These could both be repaired using the homologous chromosome, except in the case of the male X or Y chromosome.