The gene distribution of the maize genome.

Previous investigations from our laboratory showed that the genomes of plants, like those of vertebrates, are mosaics of isochores, i.e., of very long DNA segments that are compositionally homogeneous and that can be subdivided into a small number of families characterized by different GC levels (GC is the mole fraction of guanine+cytosine). Compositional DNA fractions corresponding to different isochore families were used to investigate, by hybridization with appropriate probes, the gene distribution in vertebrate genomes. Here we report such a study on the genome of a plant, maize. The gene distribution that we found is most striking, in that almost all genes are present in isochores covering an extremely narrow (1-2%) GC range and only representing 10-20% of the genome. This gene distribution, which seems to characterize other Gramineae as well, is remarkably different from the gene distribution previously found in vertebrate genomes.