| Research Abstract |
Band zones of mammalian chromosome are associated with various genome characteristics such as DNA replication timing during S phase and long-range GC% mosaic structures. Therefore, band boundaries are thought to correspond to transition regions for replication timing and GC% levels, In this study, to identify replication switch point precisely, we determined replication timing in MHC and its surrounding non-MHC regions in and around XIC (X inactivation center), and in and around PAB (pseudoautosomal boundary), after chromosome walking and sequencing ; then correlation with GC% levels and chromosome band zones was examined. Concordant transitions for replication timing and GC% level were found in the junctions between MHC classes II and III, between class I and its telomeric non-MHC, and between class II and its centromeric non-MHC.The junction between class I and its telomeric non-MHC was assigned cytogenetically as the border between 6p21.3 and p22.1. The replication switch was also found in the 3' portion of XIST locus in XIC, that is thought to correspond to the border between Xql3.2 and q13.3, Within these junction regions, there are characteristic sequences including polypurine/polypyrimidine tracts with triplex-forming potential Triplexes are known to occur readily in polypurine/polypyrimidine sequences and to pause replication-fork movement in vitro. Using immunostaining with antitriplex antibodies and non-denatured FISH 'with polypurine/polypyrimidine probes, triplexes were found to be sequence-specificaly associated spatially with individual centromeres in the human interphase nucleus.
|
