Kooter J M, Winter A J, de Oliveira C, Wagter R, Borst P
Division of Molecular Biology, The Netherlands Cancer Institute, Amsterdam.
Gene. 1988 Sep 15;69(1):1-11. doi: 10.1016/0378-1119(88)90372-1.
Active genes for variant-specific surface glycoproteins (VSGs) reside in telomeric expression sites and may be replaced by other VSG genes via telomere conversions. The availability of a complete map of expression site 221 in variant 221a made it possible to determine the boundaries of such conversions and the sequences that are involved. We have analysed five trypanosome populations that arose from variant 221a through replacement of the 221 gene by another VSG gene. In each of these relapsed populations the telomere conversion ends at a different position in the expression site. In the relapsed population, 221aR3, the boundary was found in the coding region of an expression-site-associated gene (ESAG). This ESAG-2 codes for a potential 368-aa protein of unknown function; it contains a N-terminal signal peptide for mediating transfer to the endoplasmic reticulum and six potential N-glycosylation sites. It shares these structural features with the ESAG-1 protein encoded in the same expression site. ESAG-2 is a member of a large gene family which includes non-functional genes. In 221aR3, the partial conversion of ESAG-2 by an ESAG-2-like sequence has disrupted the open reading frame. The two ESAG-2 sequences are similar (92% identity) suggesting that sequence homology between telomeres provides the opportunity for gene conversion.
变体特异性表面糖蛋白（VSG）的活性基因位于端粒表达位点，并且可能通过端粒转换被其他VSG基因取代。221a变体中表达位点221的完整图谱的可得性使得确定此类转换的边界以及所涉及的序列成为可能。我们分析了五个源自221a变体的锥虫群体，这些群体中221基因被另一个VSG基因所取代。在这些复发群体的每一个中，端粒转换在表达位点的不同位置结束。在复发群体221aR3中，边界位于一个表达位点相关基因（ESAG）的编码区域。这个ESAG-2编码一种功能未知的潜在368个氨基酸的蛋白质；它包含一个用于介导向内质网转运的N端信号肽和六个潜在的N-糖基化位点。它与同一表达位点编码的ESAG-1蛋白具有这些结构特征。ESAG-2是一个包括无功能基因的大基因家族的成员。在221aR3中，ESAG-2被一个类似ESAG-2的序列部分转换，破坏了开放阅读框。这两个ESAG-2序列相似（92%的同一性），表明端粒之间的序列同源性为基因转换提供了机会。
