Sandegren Linus, Sjöberg Britt-Marie
Department of Molecular Biology & Functional Genomics, Stockholm University, Svante Arrhenius vag 16, F3. SE-10691 Stockholm, Sweden.
J Biol Chem. 2004 May 21;279(21):22218-27. doi: 10.1074/jbc.M400929200. Epub 2004 Mar 15.
Self-splicing group I introns are being found in an increasing number of bacteriophages. Most introns contain an open reading frame coding for a homing endo-nuclease that confers mobility to both the intron and the homing endonuclease gene (HEG). The frequent occurrence of intron/HEG has raised questions whether group I introns are spread via horizontal transfer between phage populations. We have determined complete sequences for the known group I introns among T-even-like bacteriophages together with sequences of the intron-containing genes td, nrdB, and nrdD from phages with and without introns. A previously uncharacterized phage isolate, U5, is shown to contain all three introns, the only phage besides T4 found with a "full set" of these introns. Sequence analysis of td and nrdB genes from intron-containing and intronless phages provides evidence that recent horizontal transmission of introns has occurred among the phages. The fact that several of the HEGs have suffered deletions rendering them non-functional implies that the homing endonucleases are of no selective advantage to the phage and are rapidly degenerating and probably dependent upon frequent horizontal transmissions for maintenance within the phage populations. Several of the introns can home to closely related intronless phages during mixed infections. However, the efficiency of homing varies and is dependent on homology in regions flanking the intron insertion site. The occurrence of optional genes flanking the respective intron-containing gene can strongly affect the efficiency of homing. These findings give further insight into the mechanisms of propagation and evolution of group I introns among the T-even-like bacteriophages.
越来越多的噬菌体中发现了自我剪接的I类内含子。大多数内含子含有一个开放阅读框，编码一种归巢内切核酸酶，该酶赋予内含子和归巢内切核酸酶基因（HEG）移动性。内含子/HEG的频繁出现引发了关于I类内含子是否通过噬菌体群体之间的水平转移传播的问题。我们已经确定了T偶数样噬菌体中已知I类内含子的完整序列，以及来自有内含子和无内含子噬菌体的含内含子基因td、nrdB和nrdD的序列。一种先前未鉴定的噬菌体分离株U5被证明含有所有三个内含子，是除T4之外唯一发现具有这些内含子“全套”的噬菌体。对含内含子和无内含子噬菌体的td和nrdB基因进行序列分析，为噬菌体之间最近发生的内含子水平传播提供了证据。几个HEG发生了缺失，使其失去功能，这一事实意味着归巢内切核酸酶对噬菌体没有选择优势，正在迅速退化，可能依赖于频繁的水平传播以在噬菌体群体中维持。在混合感染期间，几个内含子可以归巢到密切相关的无内含子噬菌体中。然而，归巢效率各不相同，并且取决于内含子插入位点侧翼区域的同源性。各个含内含子基因侧翼的可选基因的出现会强烈影响归巢效率。这些发现进一步深入了解了I类内含子在T偶数样噬菌体中的传播和进化机制。
