Zawadzki P, Riley M A, Cohan F M
Department of Biology, Wesleyan University, Middletown, Connecticut 06459-0170, USA.
J Bacteriol. 1996 Jan;178(1):191-8. doi: 10.1128/jb.178.1.191-198.1996.
We have surveyed naturally occurring plasmids in strains of Bacillus subtilis and the closely related species B. mojavensis and B. licheniformis. Previous studies have failed to find host-benefitting functions for plasmids of these species, suggesting that these plasmids are nonmutualistic. Only one type of plasmid was found in each plasmid-bearing strain, suggesting that most of the plasmids infecting these Bacillus species are in the same incompatibility group. A sample of 18 plasmids from these species ranged in size from 6.9 to 16 kb, with all but 6 plasmids falling into three size groups. These groups differed in the sizes of their host ranges and geographical ranges. All but 1 of the 18 plasmids from these three host species are homologous with one another. The cryptic plasmids from these three species are far less diverse than are plasmids (from other species) that are known to benefit their bacterial hosts. The low-level diversity among these cryptic plasmids is consistent with the hypothesis that host-benefitting adaptations play an important role in fostering the coexistence of plasmid populations, but other explanations for the low-level plasmid diversity are possible. Comparison of the phylogenies of the plasmids with those of their hosts suggests that Bacillus plasmids are horizontally transferred in nature at a low rate similar to that found for the colicin plasmids of Escherichia coli.
我们调查了枯草芽孢杆菌菌株以及与其亲缘关系密切的莫哈韦芽孢杆菌和地衣芽孢杆菌中的天然质粒。先前的研究未能发现这些物种的质粒具有对宿主有益的功能，这表明这些质粒并非互利共生。在每个携带质粒的菌株中仅发现一种类型的质粒，这表明感染这些芽孢杆菌物种的大多数质粒属于同一不相容群。从这些物种中选取的18个质粒样本大小在6.9至16 kb之间，除6个质粒外，其余均分为三个大小组。这些组在宿主范围大小和地理范围上存在差异。来自这三个宿主物种的18个质粒中，除1个外，其余均彼此同源。与已知对其细菌宿主有益的质粒（来自其他物种）相比，这三个物种的隐蔽质粒的多样性要低得多。这些隐蔽质粒的低水平多样性与宿主有益适应性在促进质粒群体共存中起重要作用的假设一致，但对于低水平质粒多样性也可能有其他解释。将质粒的系统发育与其宿主的系统发育进行比较表明，芽孢杆菌质粒在自然界中的水平转移速率较低，类似于大肠杆菌的大肠杆菌素质粒。
