Wiley E A, Zakian V A
Fred Hutchinson Cancer Research Center, Seattle, Washington 98104.
Genetics. 1995 Jan;139(1):67-79. doi: 10.1093/genetics/139.1.67.
Yeast telomeric DNA is assembled into a nonnucleosomal chromatin structure known as the telosome, which is thought to influence the transcriptional repression of genes placed in its vicinity, a phenomenon called telomere position effect (TPE). The product of the RAP1 gene, Rap1p, is a component of the telosome. We show that the fraction of cells exhibiting TPE can be substantially reduced by expressing large amounts of a deletion derivative of Rap1p that is unable to bind DNA, called Rap1 delta BBp, or by introducing extra telomeres on a linear plasmid, presumably because both compete in trans with telomeric chromatin for factor(s) important for TPE. This reduction in TPE, observed in three different strains, was demonstrated for two different genes, each assayed at a different telomere. In contrast, the addition of internal tracts of telomeric DNA on a circular plasmid had very little effect on TPE. The product of the SIR3 gene, Sir3p, appears to be limiting for TPE. Overexpression of Sir3p completely suppressed the reduction in TPE observed with expression of Rap1 delta BBp, but did not restore high levels of TPE to cells with extra telomeres. These results suggest that extra telomeres must titrate a factor other than Sir3p that is important for TPE. These results also provide evidence for a terminus-specific binding factor that is a factor with a higher affinity for DNA termini than for nonterminal tracts of telomeric DNA and indicate that this factor is important for TPE.
酵母端粒DNA组装成一种称为端粒小体的非核小体染色质结构，这种结构被认为会影响其附近基因的转录抑制，这一现象称为端粒位置效应（TPE）。RAP1基因的产物Rap1p是端粒小体的一个组成部分。我们发现，通过表达大量无法结合DNA的Rap1p缺失衍生物（称为Rap1δBBp），或通过在线性质粒上引入额外的端粒，表现出TPE的细胞比例可大幅降低，推测这是因为两者都在反式作用中与端粒染色质竞争对TPE重要的因子。在三种不同菌株中观察到的这种TPE降低，在两个不同基因上得到了证实，每个基因在不同的端粒处进行检测。相比之下，在环状质粒上添加端粒DNA内部片段对TPE的影响很小。SIR3基因的产物Sir3p似乎是TPE的限制因素。Sir3p的过表达完全抑制了在表达Rap1δBBp时观察到的TPE降低，但没有将TPE恢复到具有额外端粒的细胞的高水平。这些结果表明，额外的端粒必须滴定一种对TPE重要的、不同于Sir3p的因子。这些结果还为一种末端特异性结合因子提供了证据，该因子对DNA末端的亲和力高于对端粒DNA非末端片段的亲和力，并表明该因子对TPE很重要。
