Bardaji Leire, Añorga Maite, Echeverría Myriam, Ramos Cayo, Murillo Jesús
1Institute for Multidisciplinary Applied Biology, Universidad Pública de Navarra, 31192 Mutilva, Spain.
2Instituto de Hortofruticultura Subtropical y Mediterránea «La Mayora», Universidad de Málaga-CSIC, Área de Genética, Universidad de Málaga, Campus de Teatinos s/n, 29010 Málaga, Spain.
Mob DNA. 2019 Jan 31;10:7. doi: 10.1186/s13100-019-0149-4. eCollection 2019.
is a γ-proteobacterium causing economically relevant diseases in practically all cultivated plants. Most isolates of this pathogen contain native plasmids collectively carrying many pathogenicity and virulence genes. However, is generally an opportunistic pathogen primarily inhabiting environmental reservoirs, which could exert a low selective pressure for virulence plasmids. Additionally, these plasmids usually contain a large proportion of repeated sequences, which could compromise plasmid integrity. Therefore, the identification of plasmid stability determinants and mechanisms to preserve virulence genes is essential to understand the evolution of this pathogen and its adaptability to agroecosystems.
The three virulence plasmids of pv. savastanoi NCPPB 3335 contain from one to seven functional stability determinants, including three highly active toxin-antitoxin systems (TA) in both pPsv48A and pPsv48C. The TA systems reduced loss frequency of pPsv48A by two orders of magnitude, whereas one of the two replicons of pPsv48C likely confers stable inheritance by itself. Notably, inactivation of the TA systems from pPsv48C exposed the plasmid to high-frequency deletions promoted by mobile genetic elements. Thus, recombination between two copies of MITE caused the deletion of an 8.3 kb fragment, with a frequency of 3.8 ± 0.3 × 10. Likewise, one-ended transposition of IS generated plasmids containing deletions of variable size, with a frequency of 5.5 ± 2.1 × 10, of which 80% had lost virulence gene . These deletion derivatives were stably maintained in the population by replication mediated by , which is adjacent to IS. IS also promoted deletions in plasmid pPsv48A, either by recombination or one-ended transposition. In all cases, functional TA systems contributed significantly to reduce the occurrence of plasmid deletions in vivo.
Virulence plasmids from harbour a diverse array of stability determinants with a variable contribution to plasmid persistence. Importantly, we showed that multiple plasmid-borne TA systems have a prominent role in preserving plasmid integrity and ensuring the maintenance of virulence genes in free-living conditions. This strategy is likely widespread amongst native plasmids of and other bacteria.
是一种γ-变形菌，可在几乎所有栽培植物中引发具有经济相关性的疾病。该病原体的大多数分离株都含有天然质粒，这些质粒共同携带许多致病和毒力基因。然而，通常是一种机会性病原体，主要栖息于环境库中，这可能对毒力质粒施加较低的选择压力。此外，这些质粒通常含有很大比例的重复序列，这可能会损害质粒的完整性。因此，鉴定质粒稳定性决定因素和保存毒力基因的机制对于理解该病原体的进化及其对农业生态系统的适应性至关重要。
pv. savastanoi NCPPB 3335的三种毒力质粒含有一到七个功能性稳定性决定因素，包括pPsv48A和pPsv48C中三个高度活跃的毒素-抗毒素系统（TA）。TA系统使pPsv48A的丢失频率降低了两个数量级，而pPsv48C的两个复制子之一可能自身赋予稳定的遗传特性。值得注意的是，pPsv48C中TA系统的失活使质粒易受移动遗传元件促进的高频缺失影响。因此，两个微型反向重复转座元件（MITE）拷贝之间的重组导致一个8.3 kb片段的缺失，频率为3.8±0.3×10。同样，插入序列（IS）的单端转座产生了含有大小可变缺失的质粒，频率为5.5±2.1×10，其中80%失去了毒力基因。这些缺失衍生物通过与IS相邻的介导的复制在群体中稳定维持。IS也通过重组或单端转座促进质粒pPsv48A中的缺失。在所有情况下，功能性TA系统在体内显著降低了质粒缺失的发生率。
来自的毒力质粒具有多种稳定性决定因素，对质粒持久性的贡献各不相同。重要的是，我们表明多个质粒携带的TA系统在维持质粒完整性和确保在自由生活条件下毒力基因的维持方面具有突出作用。这种策略可能在和其他细菌的天然质粒中广泛存在。
