Fine structure and evolution of the rDNA intergenic spacer in rice and other cereals

The intergenic spacer of a rice ribosomal RNA gene repeating unit has been completely sequenced. The spacer contains three imperfect, direct repeated regions of 264-253 bp, followed by a related but more highly divergent region. Detailed analysis of the sequence allows the presentation of an evolutionary scenario in which the 264-253-bp repeats are derived from an ancestral 150-bp sequence by deletion and amplification. Comparison of the rice sequence with those of maize, wheat, and rye shows that, despite considerable divergence from the ancestral sequence, several regions have been highly conserved, suggesting that they may play an important role in the structure and/or expression of the ribosomal genes.