Comparison of Genome Structures of Vibrios, Bacteria Possessing Two Chromosomes

Vibrios are gram-negative γ-proteobacteria which are ubiquitous in marine and estuarine environments. Recently, we demonstrated that some, if not all, Vibrio species have two circular chromosomes. The whole genome sequence of Vibrio cholerae N16961 has been reported. In this study, we constructed a physical and genetic map of the genome of Kanagawa phenomenon-positive Vibrio parahaemolyticus strain KX-V237 and compared it with those of V. parahaemolyticus AQ4673 and V. cholerae N16961. The genome of KX-V237 comprised two circular chromosomes (3.3 and 1.9 Mb), similar to the structure of the AQ4673 genome. The relative positions of the genes on the genomes were well conserved in the two strains, but a large inversion on the large chromosomes, probably symmetric around the replication origin, was suggested. Although the sizes of the large chromosomes of KX-V237 and V. cholerae N16961 were similar, the sizes of the small chromosomes were very different. Unlike N16961, the superintegron of KX-V237 was located on the large chromosome. Comparison of the genetic maps of the chromosomes of KX-V237 and V. cholerae N16961 revealed that most of the open reading frames (ORFs) present on the large chromosome of the V. cholerae strain had homologues on the large chromosome of the V. parahaemolyticus strain and that most of the ORFs on the small chromosome of N16961 were present on the small chromosome of KX-V237. The difference in the orders of the ORFs on the chromosomes of N16961 and KX-V237 implies that numerous and frequent genetic exchanges have occurred intrachromosomally rather than interchromosomally.

弧菌属（Vibrio）细菌为革兰氏阴性γ-变形菌纲（γ-proteobacteria），广泛分布于海洋与河口环境中。近期研究证实，即便并非全部，部分弧菌菌种携带有两条环状染色体。霍乱弧菌（Vibrio cholerae）N16961的全基因组序列已被公开报道。本研究中，我们构建了神奈川现象阳性（Kanagawa phenomenon-positive）副溶血性弧菌（Vibrio parahaemolyticus）菌株KX-V237的基因组物理遗传图谱，并将其与副溶血性弧菌AQ4673及霍乱弧菌N16961的对应图谱进行了比较分析。KX-V237的基因组包含两条环状染色体，大小分别为3.3 Mb与1.9 Mb，其基因组结构与AQ4673高度相似。两菌株基因组上的基因相对位置保守性良好，但推测在大型染色体上存在一处大概率围绕复制原点对称的大片段倒位事件。尽管KX-V237与霍乱弧菌N16961的大型染色体尺寸相近，二者的小型染色体尺寸却存在显著差异。与N16961不同，KX-V237的超级整合子（superintegron）定位于大型染色体上。对KX-V237与霍乱弧菌N16961的染色体遗传图谱进行比对后发现，霍乱弧菌菌株大型染色体上的绝大多数开放阅读框（open reading frames, ORFs）在副溶血性弧菌菌株的大型染色体上均存在同源基因；且N16961小型染色体上的绝大多数开放阅读框，也可在KX-V237的小型染色体中找到对应序列。N16961与KX-V237的染色体开放阅读框排列顺序存在差异，这暗示二者之间发生了大量且频繁的染色体内遗传重组，而非染色体间的遗传交换事件。