Woese C R, Winker S, Gutell R R
Department of Microbiology, University of Illinois, Urbana 61801.
Proc Natl Acad Sci U S A. 1990 Nov;87(21):8467-71. doi: 10.1073/pnas.87.21.8467.
The four-base loops that cap many double-helical structures in rRNA (the so-called "tetra-loops") exhibit highly invariant to highly variable sequences depending upon their location in the molecule. However, in the vast majority of these cases the sequence of a tetra-loop is independent of its location and conforms to one of three general motifs, GNRA, UNCG, and (more rarely) CUUG. For the most frequently varying of the 16S rRNA tetra-loops, that at position 83 (Escherichia coli numbering), the three sequences CUUG, UUCG, and GCAA account for almost all examples encountered, and each of them has independently arisen at least a dozen times. The closing base pair of tetra-loop hairpins reflects the loop sequence, tending to be C.G for UUCG loops and G.C for CUUG loops.
在rRNA中许多双螺旋结构顶端的四碱基环（即所谓的“四环”），其序列根据在分子中的位置呈现出从高度保守到高度可变的情况。然而，在绝大多数情况下，四环的序列与其位置无关，并且符合三种一般基序之一，即GNRA、UNCG和（较少见的）CUUG。对于16S rRNA中变化最频繁的四环，即位于83位（大肠杆菌编号）的四环，CUUG、UUCG和GCAA这三种序列几乎涵盖了所有已发现的例子，并且它们各自至少独立出现了十二次。四环发夹结构的末端碱基对反映了环序列，对于UUCG环往往是C.G，对于CUUG环则是G.C。
