Adams D S, Eickbush T H, Herrera R J, Lizardi P M
J Mol Biol. 1986 Feb 20;187(4):465-78. doi: 10.1016/0022-2836(86)90327-x.
A library of low Cot DNA (Cot is the molar concentration of DNA times the incubation time in seconds) from Bombyx mori was used to isolate five independent clones of highly reiterated sequences from the genome of this organism. Sequence analysis revealed that all five clones belong to a single family of repetitive DNA elements, which we have named Bm1, and whose reiteration frequency is approximately 2.3 X 10(4) copies per haploid genome. Probing of a Bombyx genomic library (in lambda phage) with a Bm1 clone reveals that this repetitive sequence is dispersed throughout the genome. The pattern of interspersion was confirmed by Southern blot mapping of a large (270 X 10(3) base-pairs) domain of the chorion locus of Bombyx, where at least 13 independent regions were found to hybridize to Bm1. Four additional Bm1 elements have been sequenced from a 4.8 X 10(3) base-pair genomic fragment containing an early chorion gene. Two of these four elements are bounded by short (4 to 12 base-pairs) direct repeats. The nine Bm1 elements which have been sequenced are greater than 88% homologous to each other, and tend to fall in at least two size classes (253 base-pairs and 450 base-pairs). Seven of the nine Bm1 elements have a short 6 to 10 base-pair oligo(A) sequence at the 3' end. A sequence of about 29 base-pairs at the 3' end, including the oligo(A), shows 86% homology to the equivalent 3'-terminal domain of human Alu family repetitive elements. A 129 base-pair domain at the 5' end of Bm1 shows 66% homology to a Drosophila valine transfer RNA gene; thus the 5' end of Bm1 may contain the split internal RNA polymerase III promoter that is characteristic of most transcribed tRNA-like retroposons. Dot-blot analysis of Bombyx RNA shows that Bm1 DNA is indeed transcribed, and that the transcripts are well-represented in the total RNA of an ovarian-derived permanent cell line and posterior silk glands early in the fifth instar, but are less abundant in the RNA of pupae or silk glands late in the fifth instar.
利用家蚕低Cot值DNA文库（Cot是DNA的摩尔浓度乘以保温时间（秒））从该生物体基因组中分离出五个高度重复序列的独立克隆。序列分析表明，所有五个克隆都属于一个重复DNA元件家族，我们将其命名为Bm1，其重复频率约为每个单倍体基因组2.3×10⁴个拷贝。用Bm1克隆对家蚕基因组文库（λ噬菌体载体）进行杂交筛选，结果表明该重复序列分散于整个基因组中。通过对家蚕卵壳基因座一个大的结构域（270×10³碱基对）进行Southern杂交图谱分析，证实了这种散布模式，在该结构域中发现至少有13个独立区域与Bm1杂交。从一个含有早期卵壳基因的4.8×10³碱基对基因组片段中又测序了另外四个Bm1元件。这四个元件中的两个由短的（4至12个碱基对）同向重复序列界定。已测序的九个Bm1元件彼此间的同源性大于88%，并且倾向于至少分为两个大小类别（253个碱基对和450个碱基对）。九个Bm1元件中的七个在3'端有一个短的6至10个碱基对的寡聚（A）序列。3'端约29个碱基对的序列，包括寡聚（A），与人Alu家族重复元件的等效3'末端结构域显示出86%的同源性。Bm1 5'端的一个129个碱基对的结构域与果蝇缬氨酸转移RNA基因显示出66%的同源性；因此，Bm1的5'端可能包含大多数转录的类tRNA反转录转座子所特有的分裂型内部RNA聚合酶III启动子。对家蚕RNA的斑点杂交分析表明，Bm1 DNA确实被转录，并且这些转录本在卵巢来源的永久细胞系和五龄初期后部丝腺的总RNA中含量丰富，但在蛹期或五龄后期丝腺的RNA中含量较少。
