Tribble G D, Parker A C, Smith C J
Department of Microbiology and Immunology, East Carolina University, Greenville, North Carolina 27858, USA.
Plasmid. 1999 Jul;42(1):1-12. doi: 10.1006/plas.1999.1401.
Tn4555 is a 12.1-kb Bacteroides antibiotic resistance transposon representative of a novel class of transmissible genetic elements that can be transferred by resident conjugative tetracycline resistance transposons (Tc(r)-elements) but are not capable of self-transfer. Previously it was shown that Tn4555 transposes by a site-specific recombination mechanism that utilizes a circular intermediate. This circular form is induced by tetracycline and it also is the substrate for conjugation. To better understand the mechanism of transposition, the entire nucleotide sequence of Tn4555 was determined and a set of genes potentially involved in transposition was identified. The transposon was 12,105 bp including a variable 6-bp coupling sequence associated with one of the transposon termini. The element had a 44.3% G + C composition and nine potential protein coding regions were observed, eight of which were encoded on the forward strand. Two putative transposition genes were found. The int gene product had significant C-terminal homology to the lambda family of integrases and the xis gene product was similar to several excisionase proteins encoded by both plasmids and conjugative transposons. The mobA mobilization gene and cfxA beta-lactamase gene of Tn4555 had been previously identified, and the remaining five open reading frames had no significant matches with sequences in the available databases. Northern hybridization analysis revealed that all Tn4555 genes except for orf-9 were expressed and two sets of genes, tnpA, int and xis, orf-5, orf-6 were organized in operons. None of the genes seemed to be induced significantly by the addition of tetracycline to cultures. Although a small 0.4-kb xis-specific transcript appeared in tetracycline-treated cultures it was not clear if this was due to an induction or if it was a specific degradation product.
Tn4555是一个12.1千碱基对的拟杆菌属抗生素抗性转座子，代表了一类新型的可传播遗传元件，它可由常驻的接合型四环素抗性转座子（Tc(r)元件）转移，但不能自我转移。先前研究表明，Tn4555通过利用环状中间体的位点特异性重组机制进行转座。这种环状形式由四环素诱导产生，它也是接合作用的底物。为了更好地理解转座机制，测定了Tn4555的完整核苷酸序列，并鉴定出一组可能参与转座的基因。该转座子为12,105碱基对，包括一个与转座子末端之一相关的可变6碱基对连接序列。该元件的G + C含量为44.3%，观察到九个潜在的蛋白质编码区，其中八个在前导链上编码。发现了两个假定的转座基因。int基因产物在C末端与整合酶的λ家族具有显著同源性，xis基因产物与质粒和接合型转座子编码的几种切除酶蛋白相似。Tn4555的mobA动员基因和cfxAβ-内酰胺酶基因先前已被鉴定，其余五个开放阅读框与现有数据库中的序列没有显著匹配。Northern杂交分析表明，除orf-9外，所有Tn4555基因均有表达，两组基因，tnpA、int和xis，orf-5、orf-6组成操纵子。向培养物中添加四环素似乎并未显著诱导任何基因。尽管在四环素处理的培养物中出现了一个小的0.4千碱基对的xis特异性转录本，但尚不清楚这是由于诱导还是特定的降解产物。
