Many bacteria utilize simple DNA sequence repeats as a mechanism to randomly switch genes on and off. This process is called phase variation. Several phase-variable N⁶-adenine DNA-methyltransferases from Type III restriction-modification systems have been reported in bacterial pathogens. Random switching of DNA methyltransferases changes the global DNA methylation pattern, leading to changes in gene expression. These epigenetic regulatory systems are called phasevarions — phase-variable regulons. The extent of these phase-variable genes in the bacterial kingdom is unknown. Here, we interrogated a database of restriction-modification systems, REBASE, by searching for all simple DNA sequence repeats in mod genes that encode Type III N⁶-adenine DNA-methyltransferases. We report that 17.4% of Type III mod genes (662/3805) contain simple sequence repeats. Of these, only one-fifth have been previously identified. The newly discovered examples are widely distributed and include many examples in opportunistic pathogens as well as in environmental species. In many cases, multiple phasevarions exist in one genome, with examples of up to 4 independent phasevarions in some species. We found several new types of phase-variable mod genes, including the first example of a phase-variable methyltransferase in pathogenic Escherichia coli. Phasevarions are a common epigenetic regulation contingency strategy used by both pathogenic and non-pathogenic bacteria.

中文翻译：

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对III型限制修饰系统的调查显示，许多新颖的表观遗传调节剂可控制相变调节子。相变

许多细菌利用简单的DNA序列重复作为随机打开和关闭基因的机制。该过程称为相位变化。在细菌病原体中，已经报道了来自III型限制性修饰系统的几种相变N ^6-腺嘌呤DNA-甲基转移酶。DNA甲基转移酶的随机切换会改变整体DNA甲基化模式，从而导致基因表达的变化。这些表观遗传调控系统称为相变-相变调节剂。这些相变基因在细菌界的范围尚不清楚。在这里，我们通过搜索编码III型N ^6的mod基因中所有简单的DNA序列重复序列，查询了限制性修饰系统REBASE的数据库。-腺嘌呤DNA-甲基转移酶。我们报告说，III型mod基因（662/3805）的17.4％包含简单的序列重复。其中，以前只确定了五分之一。新发现的例子分布广泛，包括机会病原体以及环境物种中的许多例子。在许多情况下，一个基因组中存在多个相变，例如在某些物种中多达4个独立的相变。我们发现了几种新型的相变mod基因，包括致病性大肠杆菌中相变甲基转移酶的第一个例子。相变是病原性细菌和非病原性细菌都使用的常见表观遗传调控应急策略。