Human pseudoautosomal boundary-like sequences: expression and involvement in evolutionary formation of the present-day pseudoautosomal boundary of human sex chromosomes.

The human genome is composed of long-range mosaic structures of G+C% (GC%), which are thought to be related to chromosome bands. We previously identified a boundary of Mb-level domains of GC% mosaic structures in the human major histocompatibility complex (MHC) and found in the domain boundary a sequence very similar to pseudoautosomal boundary (PAB) sequences of human sex chromosomes. We designated it 'PABL' and found many PABLs in the human genome. By analysis of six genomic and six transcribed PABLs, a core and consensus sequence of about 650 nt was defined; the 3'- and 5'-edges of the PABLs were strictly conserved. Northern blot analysis showed sizes of PABL transcripts to be 5-10 kb in length. Divergence time of PABLs was estimated to be 60-120 million years ago by analysis of human PABLs and PABXY1 of seven primates, and the evolutionary rates deduced showed PABLs to have been under selective constraints. A model for evolutionary formation of the present pseudoautosomal boundary was proposed by postulation of illegitimate recombination between two PABLs.