Meier J T, Lewis S M
Division of Biology, California Institute of Technology, Pasadena 91125.
Mol Cell Biol. 1993 Feb;13(2):1078-92. doi: 10.1128/mcb.13.2.1078-1092.1993.
Antigen receptor genes acquire junctional inserts upon assembly from their component, germ line-encoded V, D, and J segments. Inserts are generally of random sequence, but a small number of V-D, D-J, or V-J junctions are exceptional. In such junctions, one or two added base pairs inversely repeat the sequence of the abutting germ line DNA. (For example, a gene segment ending AG might acquire an insert beginning with the residues CT upon joining). It has been proposed that the nonrandom residues, termed "P nucleotides," are a consequence of an obligatory end-modification step in V(D)J recombination. P insertion in normal, unselected V(D)J joining products, however, has not been rigorously established. Here, we use an experimentally manipulable system, isolated from immune selection of any kind, to examine the fine structure of V(D)J junctions formed in wild-type lymphoid cells. Our results, according to statistical tests, show the following, (i) The frequency of P insertion is influenced by the DNA sequence of the joined ends. (ii) P inserts may be longer than two residues in length. (iii) P inserts are associated with coding ends only. Additionally, a systematic survey of published P nucleotide data shows no evidence for variation in P insertion as a function of genetic locus and ontogeny. Together, these analyses establish the generality of the P nucleotide pattern within inserts but do not fully support previous conjectures as to their origin and centrality in the joining reaction.
抗原受体基因在由其组成的种系编码的V、D和J片段组装时会获得连接插入序列。插入序列通常是随机序列，但少数V-D、D-J或V-J连接是例外情况。在这种连接中，一两个添加的碱基对会反向重复相邻种系DNA的序列。（例如，一个以AG结尾的基因片段在连接时可能会获得一个以CT开头的插入序列）。有人提出，这些非随机的残基，即“P核苷酸”，是V(D)J重组中一个强制性末端修饰步骤的结果。然而，在正常的、未经选择的V(D)J连接产物中P插入尚未得到严格证实。在这里，我们使用一个可通过实验操作的系统，该系统与任何类型的免疫选择无关，来研究野生型淋巴细胞中形成的V(D)J连接的精细结构。根据统计测试，我们的结果表明：（i）P插入的频率受连接末端DNA序列的影响。（ii）P插入的长度可能超过两个残基。（iii）P插入仅与编码末端相关。此外，对已发表的P核苷酸数据的系统调查表明，没有证据表明P插入会因基因座和个体发育而有所不同。这些分析共同确立了插入序列中P核苷酸模式的普遍性，但并未完全支持此前关于其起源及其在连接反应中的核心地位的推测。
