AMPLICONS OF MAIZE ZEIN GENES ARE CONSERVED WITHIN GENIC BUT EXPANDEDAND CONSTRICTED IN INTERGENIC REGIONS

The 78 101 base pair long sequence of a cluster of 22-kDa alpha zein g enes in the maize inbred BSSS53 was determined. Each zein gene is cont ained within a repeat unit that varies in length. If such a repeat, or amplicon, is aligned along the entire sequence, a 10.5-fold sequence amplification is delineated. Because of insertions and deletions in in tergenic regions, many of the zein genes are spaced over different dis tances. Only three out of 10 zein-related sequences have an intact ope n reading frame, indicating an unusual large number of genes unable to contribute to the accumulation of normal-size 22-kDa zein proteins. I t is proposed that the seven remaining zein-related sequences be consi dered gene reserves because of their potential to be restored by gene conversion. Intergenic insertions in the cluster range from 1098 to 14 896 base pairs. Although they are composed of transposable element se quences, they also contain additional open reading frames, two of them showing homology to rice cDNA sequences. The average amplicon is 4423 base pairs long, with the sequence surrounding each zein gene more th an 90 % conserved. Coincidently, the size of the amplicon is equivalen t to the average gene density (one gene within 4640 bp) in the Arabido psis thaliana genome, one of the smallest in plants. Successive steps of amplification and insertion of DNA might explain to a certain degre e how genome size variation has been generated in plants.