Department of Microbiology and Immunology, F, Edward Hébert School of Medicine, Uniformed Services University of the Health Sciences, 4301 Jones Bridge Road, Bethesda, MD 20814-4799, USA.
BMC Microbiol. 2009 Dec 31;9:279. doi: 10.1186/1471-2180-9-279.
rRNA adenine dimethyltransferases, represented by the Escherichia coli KsgA protein, are highly conserved phylogenetically and are generally not essential for growth. They are responsible for the post-transcriptional transfer of two methyl groups to two universally conserved adenosines located near the 3'end of the small subunit rRNA and participate in ribosome maturation. All sequenced genomes of Chlamydia reveal a ksgA homolog in each species, including C. trachomatis. Yet absence of a S-adenosyl-methionine synthetase in Chlamydia, the conserved enzyme involved in the synthesis of the methyl donor S-adenosyl-L-methionine, raises a doubt concerning the activity of the KsgA homolog in these organisms.
Lack of the dimethylated adenosines following ksgA inactivation confers resistance to kasugamycin (KSM) in E. coli. Expression of the C. trachomatis L2 KsgA ortholog restored KSM sensitivity to the E. coli ksgA mutant, suggesting that the chlamydial KsgA homolog has specific rRNA dimethylase activity. C. trachomatis growth was sensitive to KSM and we were able to isolate a KSM resistant mutant of C. trachomatis containing a frameshift mutation in ksgA, which led to the formation of a shorter protein with no activity. Growth of the C. trachomatis ksgA mutant was negatively affected in cell culture highlighting the importance of the methylase in the development of these obligate intracellular and as yet genetically intractable pathogens.
The presence of a functional rRNA dimethylase enzyme belonging to the KsgA family in Chlamydia presents an excellent chemotherapeutic target with real potential. It also confirms the existence of S-adenosyl-methionine--dependent methylation reactions in Chlamydia raising the question of how these organisms acquire this cofactor.
rRNA 腺嘌呤二甲基转移酶，以大肠杆菌 KsgA 蛋白为代表，在系统发育上高度保守，通常对生长不是必需的。它们负责将两个甲基基团在后转录水平转移到小亚基 rRNA 3'端附近的两个普遍保守的腺嘌呤上，并参与核糖体成熟。所有已测序的衣原体基因组在每个物种中都发现了一个 ksgA 同源物，包括沙眼衣原体。然而，衣原体中缺乏 S-腺苷甲硫氨酸合成酶，而 S-腺苷甲硫氨酸是合成甲基供体 S-腺苷-L-甲硫氨酸的保守酶，这使人怀疑这些生物体中 KsgA 同源物的活性。
在大肠杆菌中，当 ksgA 失活时，由于缺乏二甲基化的腺嘌呤，因此对 kasugamycin（KSM）产生抗性。沙眼衣原体 L2 KsgA 同源物的表达使 E. coli ksgA 突变体对 KSM 敏感，这表明衣原体 KsgA 同源物具有特定的 rRNA 二甲基化酶活性。沙眼衣原体的生长对 KSM 敏感，我们能够分离到一个 KSM 抗性突变体，该突变体在 ksgA 中发生了移框突变，导致形成一个没有活性的较短蛋白质。沙眼衣原体 ksgA 突变体在细胞培养中的生长受到负面影响，这突出表明该甲基化酶在这些必需的细胞内和目前遗传上难以处理的病原体的发育中非常重要。
衣原体中存在属于 KsgA 家族的功能性 rRNA 二甲基化酶，这为化学治疗提供了一个极好的靶标，具有真正的潜力。它还证实了衣原体中存在 S-腺苷甲硫氨酸依赖性甲基化反应，这引发了一个问题，即这些生物体如何获得这种辅助因子。
