Peterson S N, Bailey C C, Jensen J S, Borre M B, King E S, Bott K F, Hutchison C A
Department of Microbiology and Immunology, University of North Carolina, Chapel Hill 27599, USA.
Proc Natl Acad Sci U S A. 1995 Dec 5;92(25):11829-33. doi: 10.1073/pnas.92.25.11829.
We have characterized a family of repetitive DNA elements with homology to the MgPa cellular adhesion operon of Mycoplasma genitalium, a bacterium that has the smallest known genome of any free-living organism. One element, 2272 bp in length and flanked by DNA with no homology to MgPa, was completely sequenced. At least four others were partially sequenced. The complete element is a composite of six regions. Five of these regions show sequence similarity with nonadjacent segments of genes of the MgPa operon. The sixth region, located near the center of the element, is an A+T-rich sequence that has only been found in this repeat family. Open reading frames are present within the five individual regions showing sequence homology to MgPa and the adjacent open reading frame 3 (ORF3) gene. However, termination codons are found between adjacent regions of homology to the MgPa operon and in the A+T-rich sequence. Thus, these repetitive elements do not appear to be directly expressible protein coding sequences. The sequence of one region from five different repetitive elements was compared with the homologous region of the MgPa gene from the type strain G37 and four newly isolated M. genitalium strains. Recombination between repetitive elements of strain G37 and the MgPa operon can explain the majority of polymorphisms within our partial sequences of the MgPa genes of the new isolates. Therefore, we propose that the repetitive elements of M. genitalium provide a reservoir of sequence that contributes to antigenic variation in proteins of the MgPa cellular adhesion operon.
我们鉴定了一类重复DNA元件家族，它们与生殖支原体的MgPa细胞黏附操纵子具有同源性，生殖支原体是一种自由生活的生物体，其基因组是已知最小的。其中一个元件长度为2272 bp，两侧是与MgPa无同源性的DNA，已完成全序列测定。另外至少四个元件进行了部分序列测定。完整的元件由六个区域组成。其中五个区域与MgPa操纵子基因的不相邻片段具有序列相似性。第六个区域位于元件中心附近，是一个富含A+T的序列，仅在这个重复家族中发现。在与MgPa具有序列同源性的五个单独区域以及相邻的开放阅读框3（ORF3）基因中存在开放阅读框。然而，在与MgPa操纵子同源的相邻区域之间以及富含A+T的序列中发现了终止密码子。因此，这些重复元件似乎不是直接可表达的蛋白质编码序列。将来自五个不同重复元件的一个区域的序列与来自模式菌株G37和四个新分离的生殖支原体菌株的MgPa基因的同源区域进行了比较。菌株G37的重复元件与MgPa操纵子之间的重组可以解释我们新分离株MgPa基因部分序列中的大多数多态性。因此，我们提出生殖支原体的重复元件提供了一个序列库，有助于MgPa细胞黏附操纵子蛋白质的抗原变异。
