REGULATION OF THE REPLICATION OF THE MURINE IMMUNOGLOBULIN HEAVY-CHAIN GENE LOCUS - EVALUATION OF THE ROLE OF THE 3'-REGULATORY REGION

DNA replication in mammalian cells is a precisely controlled physical and temporal process, likely involving cis-acting elements that contro l the region(s) from which replication initiates. In B cells, previous studies showed replication timing to be early throughout the immunogl obulin heavy chain (Igh) locus. The implication from replication timin g studies in the B-cell line MPC11 was that early replication of the I gh locus was regulated by sequences downstream of the C alpha gene. A potential candidate for these replication control sequences was the 3' regulatory region of the Igh locus. Our results demonstrate, however, that the Igh locus maintains early replication in a B-cell line in wh ich the 3' regulatory region has been deleted from one allele, thus in dicating that replication timing of the locus is independent of this r egion. In non-B cells (murine erythroleukemia cells [MEL]), previous s tudies of segments within the mouse Igh locus demonstrated that DNA re plication likely initiated downstream of the Igh gene cluster. Here we use recently cloned DNA to demonstrate that segments located sequenti ally downstream of the Igh 3' regulatory region continue to replicate progressively earlier in S phase in MEL. Furthermore, analysis by two- dimensional gel electrophoresis indicates that replication forks proce ed exclusively in the 3'-to-5' direction through the region 3' of the Igh locus. Extrapolation from these data predicts that initiation of D NA replication occurs in MEL at one or more sites within a 90-kb inter val located between 40 and 130 kb downstream of the 3' regulatory regi on.