Jin Y K, Bennetzen J L
Department of Biological Sciences, Purdue University, West Lafayette, IN 47907.
Proc Natl Acad Sci U S A. 1989 Aug;86(16):6235-9. doi: 10.1073/pnas.86.16.6235.
We have sequenced Bs1, an insertion element isolated from a null allele of the Adh1 locus encoding alcohol dehydrogenase in maize. The Bs1 element is 3203 base pairs (bp) in length, has 302-bp identical long terminal direct repeats (LTRs), and created a 5-bp flanking direct duplication of target Adh1 DNA upon insertion. The 5' LTR is followed by a canonical primer binding site with homology to the plant initiator methionyl-tRNA, and the 3' LTR is directly preceded by a polypurine stretch like that observed in retroviruses and retrotransposons. Bs1 encodes two overlapping open reading frames specifying peptides of 740 and 168 amino acids. The longer open reading frame specifies a peptide with amino acid homology to the protease and nucleic acid binding moiety of retroviruses and retrotransposons. The deduced amino acid sequence encoded by Bs1 lacks convincing homology to the polymerase (reverse transcriptase) encoded by retroposons, despite the fact that this polymerase-encoding domain is routinely the most conserved region of any such element. The sequence and relatively small size of Bs1 suggest that this element is a deleted retrotransposon that inserted into Adh1 with the aid of a reverse transcriptase function provided in trans. In vitro transcribed Bs1 complementary RNA was translated in vitro to produce both a protein of 81 kDa representing open reading frame 1 (ORF1) and one of the 95-kDa size predicted for the frame-shifted fusion of ORF1 and ORF2. As with many other retroposons, the efficiency of translational initiation at the AUG beginning ORF1 was not noticeably affected by the presence of one or more upstream, unproductive AUGs in the complementary RNA transcript.
我们已对Bs1进行了测序，Bs1是从玉米中编码乙醇脱氢酶的Adh1基因座的一个无效等位基因中分离出的一个插入元件。Bs1元件长度为3203个碱基对（bp），具有302 bp相同的长末端正向重复序列（LTRs），并在插入时在目标Adh1 DNA两侧产生了一个5 bp的侧翼正向重复序列。5' LTR之后是一个与植物起始甲硫氨酰 - tRNA具有同源性的典型引物结合位点，3' LTR之前直接是一个多聚嘌呤序列，类似于在逆转录病毒和逆转座子中观察到的那样。Bs1编码两个重叠的开放阅读框，分别指定740和168个氨基酸的肽段。较长的开放阅读框指定了一个与逆转录病毒和逆转座子的蛋白酶及核酸结合部分具有氨基酸同源性的肽段。尽管逆转座子编码的聚合酶（逆转录酶）结构域通常是任何此类元件中最保守的区域，但Bs1推导的氨基酸序列与逆转座子编码的聚合酶缺乏令人信服的同源性。Bs1的序列和相对较小的尺寸表明，该元件是一个缺失的逆转座子，它借助反式提供的逆转录酶功能插入到Adh1中。体外转录的Bs1互补RNA在体外进行翻译，产生了一个代表开放阅读框1（ORF1）的81 kDa蛋白质和一个为ORF1与ORF2移码融合预测的95 kDa大小的蛋白质。与许多其他逆转座子一样，互补RNA转录本中一个或多个上游非生产性AUG的存在并未显著影响在起始ORF1的AUG处的翻译起始效率。
