The structure of SH2/A1-homologous regions in maize, rice and sorghum.

摘要

Cereal genomes have similar gene contents and regions of colinear gene order. Microcolinearity, when present, could allow map-based cloning of agronomically important genes from large and small genome species. The sh2 and a1 genes in maize are about 0.1 cM and 140 kb apart. We have cloned the orthologous regions from rice (indica and japonica) and sorghum. The presence, order and transcriptional orientation of sh2 and a1 homologues are conserved among maize, rice and sorghum, but the physical distance between sh2 and a1 homologues is only about 20 kb in both rice and sorghum. We have completely sequenced the sh2/ a1-homologous regions from indica rice (30,035 bp), japonica rice (29,877 bp) and sorghum (42,446 bp). A putative transcriptional regulatory gene X was found between sh2 and a1 homologues in both rice and sorghum. Comparison of colinear regions of rice and sorghum indicates that only the homologues of sh2, a1 and gene X are conserved, while comparison of orthologous regions of indica and japonica rice reveals that sequence homology is 99% across the two regions. The rate of divergence in the intergenic region is more than 3-fold higher than the rate of divergence in introns and synonymous substitutions within exons. Specific regions of the intergenic space, notably those containing miniature inverted repeat transposable elements (MITEs), evolve very rapidly, with divergence frequencies over 9-fold higher than that of introns or exons in this same region.