Llaca V, Messing J
Waksman Institute, Rutgers, State University of New Jersey, Piscataway 08855, USA.
Plant J. 1998 Jul;15(2):211-20. doi: 10.1046/j.1365-313x.1998.00200.x.
The 78,101 base pair long sequence of a cluster of 22-kDa alpha zein genes in the maize inbred BSSS53 was determined. Each zein gene is contained 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 intergenic regions, many of the zein genes are spaced over different distances. Only three out of 10 zein-related sequences have an intact open reading frame, indicating an unusual large number of genes unable to contribute to the accumulation of normal-size 22-kDa zein proteins. It is proposed that the seven remaining zein-related sequences be considered 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 sequences, 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 than 90% conserved. Coincidently, the size of the amplicon is equivalent to the average gene density (one gene within 4640 bp) in the Arabidopsis thaliana genome, one of the smallest in plants. Successive steps of amplification and insertion of DNA might explain to a certain degree how genome size variation has been generated in plants.
测定了玉米自交系BSSS53中22-kDaα-玉米醇溶蛋白基因簇的78101个碱基对长的序列。每个玉米醇溶蛋白基因都包含在一个长度可变的重复单元中。如果将这样一个重复序列或扩增子沿着整个序列排列，就会勾勒出10.5倍的序列扩增。由于基因间区域的插入和缺失，许多玉米醇溶蛋白基因的间距不同。10个与玉米醇溶蛋白相关的序列中只有3个具有完整的开放阅读框，这表明有异常大量的基因无法促进正常大小的22-kDa玉米醇溶蛋白的积累。有人提出，其余7个与玉米醇溶蛋白相关的序列应被视为基因储备，因为它们有可能通过基因转换得以恢复。该基因簇中的基因间插入序列长度从1098到14896个碱基对不等。尽管它们由转座元件序列组成，但也包含额外的开放阅读框，其中两个与水稻cDNA序列具有同源性。平均扩增子长度为4423个碱基对，每个玉米醇溶蛋白基因周围的序列保守性超过90%。巧合的是，扩增子的大小与拟南芥基因组中的平均基因密度（4640 bp中有一个基因）相当，拟南芥基因组是植物中最小的基因组之一。DNA的连续扩增和插入步骤可能在一定程度上解释了植物基因组大小变异是如何产生的。
