PFGE analysis of the rice genome: estimation of fragment sizes, organization of repetitive sequences and relationships between genetic and physical distances.

Pulsed-field gel electrophoresis (PFGE) has been applied to analyze the rice nuclear genome. Probing 56 RFLP probes selected from the 12 rice chromosomes to PFGE blots of nine rare-cutting restriction enzymes revealed that there are relatively high numbers of 'rare-cutting' restriction sites in the rice genome. The average sizes of restriction fragments detected by single-copy probes are smaller than 200 kb for all of the rare-cutting restriction enzymes examined. Sizes of fragments detected by repetitive probes are variable, depending on the probes analyzed. By using PFGE, a tandemly repeated sequence, Os48, was found to be tightly linked to telomeric tandem repeats but not physically linked to r5s genes with which sequence homology had been observed. Relationships between genetic and physical distances have been established for three different chromosomal segments. In these regions 1 cm corresponds to ca. 260 kb on average. Analysis of a cluster of RFLP markers on chromosome 3 revealed that genetically clustered RFLP markers are also physically closely linked, suggesting that clustering of genetic markers may result in part from uneven distribution of single-copy sequences.