Phenotypic diversity mediated by the maize transposable elements Ac and Spm

Mutations caused by the insertion of members of the Ac or Spm family of transposable elements result in a great diversity of phenotypes. With the cloning of the mutant genes and the characterization of their products, the mechanisms underlying phenotypic diversity are being deciphered. These mechanisms include (i) imprecise excision of transposable elements, which can result in the addition of amino acids to proteins; (ii) DNA methylation, which has been correlated with the activity of the element; (iii) transposase-mediated deletions within elements, which can inactivate an element or lead to a new unstable phenotype; and (iv) removal of transcribed elements from RNA, which can facilitate gene expression despite the insertion of elements into exons. An understanding of the behavior of the maize elements has provided clues to the function of cryptic elements in all maize genomes.     

A single genetic unit specifies two transposition functions in the maize element activator

The self-mobile maize transposable element Ac (Activator) displays two trans-acting genetic functions: it induces transposition of the element Ds (Dissociation) but, as its dosage is increased, it also inhibits transposition. Previous work has shown that the 4563 base pair (bp)-long Ac element contains three open reading frames (ORF's) and that a deletion in ORF 1 in wx-m9(Ds), a Ds derivative from Ac isolated at the wx (waxy) locus, results in loss of transposition. The Ds element in the bronze allele bz-m2(DI) is shown to have arisen from Ac by a 1312-bp deletion that is located almost entirely within ORF 2 and does not affect ORF 1. The Ds elements in wx-m9(Ds) and bzm2(DI), defective in ORF 1 and ORF 2, respectively, do not complement genetically to restore the transposition function of Ac; therefore, this function must be specified jointly by ORF's 1 and 2. Furthermore, since bz-m2(DI) does not contribute to Ac's inhibitory dosage effect, both Ac properties result from the expression of the same genetic functional unit.     

Probing the component structure of a maize gene with transposable elements

Three instances of R gene instability were found in maize stocks carrying the controlling elements Dissociation (Ds) and Modulator (Mp). In each, Ds or a Ds-like element had transposed to R, inhibiting kernel pigmentation irregularly. When Mp was removed from the genome, R expression stabilized at lowt to intermediate levels. Strong pigmenting action was restored through recombination in heterozygotes of the three new forms with an R allele that specifies only plant pigmentation. The sites of Ds insertion mapped distal to the region that specifies seed versus plant expression. The evidence suggests that an R functional unit consists of one component that both governs tissue-specific expression and another that is common to alleles of different tissue-specific activities.     

Pesticide resistance via transposition-mediated adaptive gene truncation in Drosophila

To study adaptation, it is essential to identify multiple adaptive mutations and to characterize their molecular, phenotypic, selective, and ecological consequences. Here we describe a genomic screen for adaptive insertions of transposable elements in Drosophila. Using a pilot application of this screen, we have identified an adaptive transposable element insertion, which truncates a gene and apparently generates a functional protein in the process. The insertion of this transposable element confers increased resistance to an organophosphate pesticide and has spread in D. melanogaster recently.     

The origins of gene instability in yeast

Two unstable mutations at the his4 locus of yeast are due to the insertion of the transposable elements Ty912 and Ty917 into the his4 regulatory region. The two transposons are related, one being derived from the other by a substitution of 4000 base pairs of DNA. Element Ty912 includes identical terminal repeats, whereas the terminal repeats of Ty917 are not identical. Transposition of Ty912 or Ty917 generates 5-base-pair duplications of the target DNA at either end of the element. Expression and reversion of a his4 gene containing Ty912 or Ty917 is controlled by three unlinked regulatory genes. The properties of these regulatory genes are similar to those described for the controlling elements in maize.     

植物水平基因转移研究进展

水平基因转移,一般分为细胞内部或者跨越物种边界的遗传物质交流。跨界直接介导方式,包括共生、内共生、寄生、嫁接等。细胞内的基因转移,主要包括细胞核与细胞器基因组间的相互渗透;跨越物种边界的遗传物质交流,主要涉及寄生与寄主植物的基因横向转移,寄主与寄生植物mRNA也会发生大规模的水平转移。基于基因组学研究进展,本研究综述了植物水平基因转移的迁移序列类型、迁移方向及迁移机制:首先,植物细胞的线粒体基因组能够整合细胞核转座元件以及叶绿体起源的tRNA基因,线粒体和叶绿体基因组的功能基因及间区序列能够迁移到核基因组;其次,植物种间,通过寄生、嫁接等方式转移大量的DNA(如线粒体基因、叶绿体基因和转座元件)和RNA(如mRNA)序列;迁移机制涉及到DNA介导和RNA介导方式,迁移方向包括单向和双向转移。迁移序列的基因功能活性研究是重要的后续研究方向。     

Identification of a chromosome 18q gene that is altered in colorectal cancers

Allelic deletions involving chromosome 18q occur in more than 70 percent of colorectal cancers. Such deletions are thought to signal the existence of a tumor suppressor gene in the affected region, but until now a candidate suppressor gene on this chromosomal arm had not been identified. A contiguous stretch of DNA comprising 370 kilobase pairs (kb) has now been cloned from a region of chromosome 18q suspected to reside near this gene. Potential exons in the 370-kb region were defined by human-rodent sequence identities, and the expression of potential exons was assessed by an "exon-connection" strategy based on the polymerase chain reaction. Expressed exons were used as probes for cDNA screening to obtain clones that encoded a portion of a gene termed DCC; this cDNA was encoded by at least eight exons within the 370-kb genomic region. The predicted amino acid sequence of the cDNA specified a protein with sequence similarity to neural cell adhesion molecules and other related cell surface glycoproteins. While the DCC gene was expressed in most normal tissues, including colonic mucosa, its expression was greatly reduced or absent in most colorectal carcinomas tested. Somatic mutations within the DCC gene observed in colorectal cancers included a homozygous deletion of the 5' end of the gene, a point mutation within one of the introns, and ten examples of DNA insertions within a 0.17-kb fragment immediately downstream of one of the exons. The DCC gene may play a role in the pathogenesis of human colorectal neoplasia, perhaps through alteration of the normal cell-cell interactions controlling growth.     

Discovery of transposable element activity among progeny of tissue culture--derived maize plants

Tissue culture-derived plants of many species have often been observed to possess both genetic and cytogenetic abnormalities. A high frequency of structurally altered chromosomes in maize (Zea mays L.) plants regenerated from tissue culture led to the prediction that newly activated transposable elements could be detected in regenerated plants. Testcrosses of 1200 progeny from 301 regenerated maize plants confirmed that ten regenerated plants from two independent embryo cell lines contained an active Actransposable element. No active Ac elements were present in the explant sources. Recovery of transposable element activity in regenerated plants indicates that some tissue culture-derived genetic variability may be the result of insertion or excision of transposable elements, or both.     

农杆菌介导Cry1 Ab/Ac抗虫转基因玉米植株的获得

转基因技术的迅速发展有效地打破了物种间的遗传壁垒,从而加快了优良基因定向聚合的进程,然而遗传转化率低及转基因安全问题成为制约其进一步发展的限制因素。基于此,本研究以玉米自交系H99为材料,将Ac/Ds双元表达载体(包含抗虫Cry1 Ab/Ac基因和gfp报告基因等)通过农杆菌介导法转入由110粒玉米幼胚诱导出的愈伤组织中,获得28株抗性转基因植株。经PCR及RT-PCR的验证结果显示,其中8株为含有Cry1 Ab/Ac目的基因且该基因有效表达的阳性植株,转化效率达7%。通过农杆菌介导法所获得的玉米转基因植株为今后剔除抗生素筛选标记,从而获得安全的玉米抗虫新种质提供了遗传材料。     

Isolation of human transcribed sequences from human-rodent somatic cell hybrids

A method was developed for selectively isolating genes from localized regions of the human genome that are contained in interspecific hybrid cells. Complementary human DNA was prepared from a human-rodent somatic cell hybrid that contained less than 1% human DNA, by using consensus 5' intron splice sequences as primers. These primers would select immature, unspliced messenger RNA (still retaining species-specific repeat sequences) as templates. Screening a derived complementary DNA library for human repeat sequences resulted in the isolation of human clones at the anticipated frequency with characteristics expected of exons of transcribed human genes--single copy sequences that hybridized to discrete bands on Northern (RNA) blots.