Conservation of microstructure between a sequenced region of the genome ofrice and multiple segments of the genome of Arabidopsis thaliana

The nucleotide sequence was determined for a 340-kb segment of rice chromos ome 2, revealing 56 putative protein-coding genes. This represents a densit y of one gene per 6.1 kb, which is higher than was reported for a previousl y sequenced segment of the rice genome. Sixteen of the putative genes were supported by matches to ESTs. The predicted products of 29 of the putative genes showed similarity to known proteins, and a further 17 genes showed si milarity only to predicted or hypothetical proteins identified in genome se quence data. The region contains a few transposable elements: one retrotran sposon, and one transposon. The segment of the rice genome studied had prev iously been identified as representing a part of rice chromosome 2 that may be homologous to a segment of Arabidopsis chromosome 4. We confirmed the c onservation of gene content and order between the two genome segments. In a ddition, we identified a further four segments of the Arabidopsis genome th at contain conserved gene content and order. In total, 22 of the 56 genes i dentified in the rice genome segment were represented in this set of Arabid opsis genome segments, with at least five genes present, in conserved order , in each segment. These data are consistent with the hypothesis that the A rabidopsis genome has undergone multiple duplication events. Our results de monstrate that conservation of the genome microstructure can be identified even between monocot and dicot species. However, the frequent occurrence of duplication, and subsequent microstructure divergence, within plant genome s may necessitate the integration of subsets of genes present in multiple r edundant segments to deduce evolutionary relationships and identify ortholo gous genes.