The bacterium Oecophyllibacter saccharovorans of family Acetobacteraceae is a symbiont of weaver ant Oecophylla smaragdina. In our previous study, we published the finding of novel O. saccharovorans strains Ha5^T, Ta1 and Jb2 (Chua et al. 2020) but their plasmid sequences have not been reported before. Here, we demonstrate for the first time that the sole rrn operon of their genomes was detected on a 6.6 kb circular replicon. This replicon occurred in high copy number, much smaller size and lower G + C content than the main chromosome. Based on these features, the 6.6 kb circular replicon was regarded as rrn operon-containing plasmid. Further restriction analysis on the plasmids confirmed their circular conformation. A Southern hybridization analysis also corroborated the presence of 16S rRNA gene and thus the rrn operon on a single locus in the genome of the O. saccharovorans strains. However, similar genome architecture was not observed in other closely related bacterial strains. Additional survey also detected no plasmid-borne rrn operon in available genomes of validly described taxa of family Acetobacteraceae. To date, plasmid localization of rrn operon is rarely documented. This study reports the occurrence of rrn operon on the smallest bacterial plasmid in three O. saccharovorans strains and discusses its possible importance in enhancing their competitive fitness as bacterial symbiont of O. smaragdina.

细菌Oecophyllibacter saccharovorans家庭Acetobacteraceae是织叶蚁的共生体黄猄蚁。在我们之前的研究中，我们发表了新的O. saccharovorans菌株 Ha5 ^T、Ta1 和 Jb2 的发现（Chua et al. 2020），但之前没有报道过它们的质粒序列。在这里，我们首次证明他们基因组的唯一rrn操纵子是在 6.6 kb 圆形复制子上检测到的。该复制子以高拷贝数、小得多的大小和比主染色体低的G+C含量出现。基于这些特征，6.6 kb 的圆形复制子被认为是rrn含有操纵子的质粒。对质粒的进一步限制分析证实了它们的环状构象。Southern 杂交分析也证实了 16S rRNA 基因的存在，因此在O. saccharovorans菌株基因组中的单个基因座上存在rrn操纵子。然而，在其他密切相关的细菌菌株中未观察到类似的基因组结构。额外的调查也没有在可有效描述的醋杆菌科分类群的可用基因组中检测到质粒携带的rrn操纵子。迄今为止，很少记录rrn操纵子的质粒定位。该研究报告了rrn操纵子在三个最小细菌质粒上的发生O. saccharovorans菌株并讨论其作为O. smaragdina细菌共生体在增强其竞争适应性方面可能的重要性。