Tumor induction by the LTR,v-src,LTR DNA in four B (MHC) congenic lines of chickens

We report that the cloned DNA harboring the long terminal repeat (LTR), v-src, LTR proviral structure is tumorigenic in chickens of the Prague congenic lines. The growth rate of these tumors is by far the highest in the recombinant CC.R1 line, the B haplotype of which is composed of the B-F/L ⁴ and B-G ¹² subregions originating from different naturally occurring haplotypes. Some of the tumors induced by the LTR, v-src, LTR DNA are repeatedly transplantable in syngeneic chickens, maintain unaltered provirus, and express v-src mRNA. Differences in the response to challenge with Rous sarcoma virus (RSV) and LTR, v-src, LTR DNA on a given experimental model are compared and possible involvement of an interaction between B-F/L and B-G region gene is considered. Regression of the LTR, v-src, LTR DNA-induced tumors did not prevent the formation and growth of tumors induced subsequently by RSV.     

The diversity of LTR retrotransposons

Eukaryotic genomes are full of long terminal repeat (LTR) retrotransposons. Although most LTR retrotransposons have common structural features and encode similar genes, there is nonetheless considerable diversity in their genomic organization, reflecting the different strategies they use to proliferate within the genomes of their hosts.     

The analysis of Circe, an LTR retrotransposon of Drosophila melanogaster, suggests that an insertion of non-LTR retrotransposons into LTR elements can create chimeric retroelements

Circe is a transposable element recently identified in Drosophila melanogaster which appears to be mostly associated with the constitutive heterochromatin. This element shows the structural features of a long terminal repeat (LTR)-containing retrotransposon: It is flanked by 240-bp-long terminal repeats, and its two open reading frames encode putative proteins resembling the gag and pol polyproteins of retroviruses. However, Circe displays striking similarities of both LOA and Ulysses, a non-LTR element and an LTR element, respectively. The result of its phylogenetic and structural analysis has allowed us to propose a new mechanism for non-LTR retrotransposon evolution.     

Morgane, a new LTR retrotransposon group, and its subfamilies in wheats

Transposable elements are the main components of grass genomes, especially in Triticeae species. In a previous analysis, we identified a very short element, Morgane_CR626934-1; here we describe more precisely this unusual element. Morgane_CR626934-1 shows high sequence identity (until 98%) with ESTs belonging to other possible small elements, expressed under abiotic and biotic stress conditions. No putative functional polyprotein could be identified in all of these different Morgane-like sequences. Moreover, elements from the Morgane_CR626934-1 subfamily are found only in wheats and Agropyrum genomes and among these species, only Ae. tauschii and T. aestivum present a high copy number of these elements. They are highly conserved in wheat genomes (95.5%). Based on the uncommon characteristics of the described Morgane-like elements, we proposed to classify them in a new group within the Class I LTR retrotransposon, the Morgane group. Electronic Supplementary Material Supplementary material is available for this article at     

LTR Retrotransposon-Gene Associations in Drosophila melanogaster

Thirty-three percent (228/682) of all long terminal repeat (LTR) retrotransposon sequences (LRSs) present in the sequenced Drosophila melanogaster genome were found to be located in or within 1000 bp of a gene. Recently inserted LTR retrotransposons are significantly more likely to be located in or within genes than are older, fragmented LTR retrotransposon sequences, indicating that most LRS-gene associations are selected against over evolutionary time. LRSs associated with conserved genes (homologenes) are especially prone to negative selection. In contrast, fragmented LRSs that have persisted in the genome over long spans of evolutionary time are preferentially associated with genes involved in signal transduction and other newly evolved functions. Reviewing Editor: Dr. Juergen Brosius     

植物LTR反转录转座子的研究进展

反转录转座子是真核生物基因组中普遍存在的一类可移动的遗传因子,它们以RNA为媒介,在基因组中不断自我复制。在高等植物中,反转录转座子是基因组的重要成分之一。反转录转座子可以分为5大类型,其中以长末端重复（LTR）类型报道较多。LTR类型由于其首尾具有长末端重复序列,内部含有PBS、PPT、GAG和POL开放阅读框、TSD等结构,可以采用生物信息学软件进行预测。LTR反转录转座子的活性受到自身甲基化和环境因素的影响,DNA甲基化抑制反转录转座子转座,而外界环境的刺激能够激活转座子,从而影响插入位点周边基因的表达。同时由于LTR反转录转座子在植物中普遍存在,丰富的拷贝数以及多态性为新型分子标记（RBIP、SSAP、IRAP、REMAP）的开发提供了良好的素材。该文对近年来国内外有关植物反转录转座子的类型、结构特征、 LTR反转录转座子的活性及其影响因素、 LTR反转录转座子的预测以及标记开发等方面的研究进展进行综述。     

植物活性长末端重复序列反转录转座子研究进展

长末端重复序列(Long terminal repeat,LTR)反转录转座子是真核生物基因组中普遍存在的一类可移动的DNA序列,它们以RNA为媒介,通过"复制粘贴"机制在基因组中不断自我复制。在高等植物中,许多活性的LTR反转录转座子已被详尽研究并应用于分子标记技术、基因标签、插入型突变及基因功能等分析。本文对植物活性LTR反转录转座子进行全面的调查,并对其结构、拷贝数和分布以及转座特性进行系统的归纳,分析了植物活性LTR反转录转座子的gag(种属特异抗原)和pol(聚合酶)序列特征,以及LTR序列中顺式调控元件的分布。研究发现自主有活性的LTR反转录转座子必须具备LTR区域以及编码Gag、Pr、Int、Rt和Rh蛋白的基因区。其中两端LTR区域具有高度同源性且富含顺式调控元件;Rt蛋白必备RVT结构域;Rh蛋白必备RNase_H1_RT结构域。这些结果为后续植物活性LTR反转录转座子的鉴定和功能分析奠定了重要基础。     

Long terminal repeat (LTR) sequences, env, and a region near the 5'' LTR influence the pathogenic potential of recombinants between Rous-associated virus types 0 and 1.

A series of recombinants between Rous-associated virus type 0 (RAV-0), RAV-1, and a replication-competent avian leukosis virus vector (RCAN) have been tested for disease potential in day-old inoculated K28 chicks. RAV-0 is a benign virus, whereas RAV-1 and RCAN induce lymphoma and a low incidence of a variety of other neoplasms. The results of the oncogenicity tests indicate that (i) the long terminal repeat regions of RAV-1 and RCAN play a major role in disease potential, (ii) subgroup A envelope glycoproteins are associated with a two- to fourfold higher incidence of lymphoma than subgroup E glycoproteins, and (iii) certain combinations of 5' viral and env sequences cause osteopetrosis in a highly context-dependent manner. Long terminal repeat and env sequences appeared to influence lymphomogenic potential by determining the extent of bursal infection within the first 2 to 3 weeks of life. This would suggest that bursal but not postbursal stem cells are targets for avian leukosis virus-induced lymphomogenesis. The induction of neutralizing antibody had no obvious influence on the incidence of lymphoma.     

A highly conserved, small LTR retrotransposon that preferentially targets genes in grass genomes

LTR retrotransposons are often the most abundant components of plant genomes and can impact gene and genome evolution. Most reported LTR retrotransposons are large elements (>4 kb) and are most often found in heterochromatic (gene poor) regions. We report the smallest LTR retrotransposon found to date, only 292 bp. The element is found in rice, maize, sorghum and other grass genomes, which indicates that it was present in the ancestor of grass species, at least 50-80 MYA. Estimated insertion times, comparisons between sequenced rice lines, and mRNA data indicate that this element may still be active in some genomes. Unlike other LTR retrotransposons, the small LTR retrotransposons (SMARTs) are distributed throughout the genomes and are often located within or near genes with insertion patterns similar to MITEs (miniature inverted repeat transposable elements). Our data suggests that insertions of SMARTs into or near genes can, in a few instances, alter both gene structures and gene expression. Further evidence for a role in regulating gene expression, SMART-specific small RNAs (sRNAs) were identified that may be involved in gene regulation. Thus, SMARTs may have played an important role in genome evolution and genic innovation and may provide a valuable tool for gene tagging systems in grass.