NotI flanking sequences: a tool for gene discovery and verification of the human genome.

A set of 22 551 unique human Notl flanking sequences (16.2 Mb) was generated. More than 40% of the set had regions with significant similarity to known proteins and expressed sequences. The data demonstrate that regions flanking Notl sites are less likely to form nucleosomes efficiently and resemble promoter regions. The draft human genome sequence contained 55.7% of the Notl flanking sequences, Celera's database contained matches to 57.2% of the clones and all public databases (including non-human and previously sequenced Notl flanks) matched 89.2% of the Notl flanking sequences (identity greater than or equal to90% over at least 50 bp, data from December 2001). The data suggest that the shotgun sequencing approach used to generate the draft human genome sequence resulted in a bias against cloning and sequencing of Notl flanks. A rough estimation (based primarily on chromosomes 21 and 22) is that the human genome contains 15 000-20 000 Notl sites, of which 6000-9000 are unmethylated in any particular cell. The results of the study suggest that the existing tools for computational determination of CpG islands fail to identify a significant fraction of functional CpG islands, and unmethylated DNA stretches with a high frequency of CpG dinucleotides can be found even in regions with low CG content.