Page 171 - 《水产学报》2025年第5期

车行，等 水产学报, 2025, 49(5): 059114

was identified in strain WC1535 isolated from Oreochromis niloticus, named ICE_Sag1535_mutT (abbreviation as
ICESag1535). The bioinformatic characteristics of ICEs were analyzed, including their conjugation modules, recombinase com-
position, variable region function, and ICE homology. ICESag1535 belongs to the ICESa2603 family-like ICEs, with a length
of approximately 74.1 kb, encoding 73 genes. It contains 25 core mobile genes of the Conj Tn5252 superfamily that form the back-
bone of conjugant modules. The conserved backbone region of ICESag1535 includes 5 foreign gene insertion hotspots (HS),
and a variable region (VR) containing 7 transposable insertion elements (IS), forming 3 complex transposons. These HS and
VR can be divided into 7 major functional regions involved in the ICE functions, such as regulation of ICE stability, conjuga-
tion coupling, material transmembrane transportation, stress regulation, bacteriocin synthesis and export, integrative and con-
jugative evolution, host adhesion. Additionally, the 3'-terminal of ICESag1535 consists of three structurally similar tandem site-
specific serine integrases (TSPSI). Cluster analysis of TSPSI revealed that S. agalactiae isolated from Oreochromis niloticus,
Lithobates catesbeiana and Lates calcarifer share identical TSPSIs within ICESa2603-like ICEs distributed in the Northern
hemisphere aquatic ecosystem. Homology analysis of ICEs indicated that the conserved conjugational transfer backbone of
ICESa2603-like ICEs, encoded by human, pig, and fish isolates, is homologous to Conj Tn5252 conjugant modules and has the
ability to spread across species. The prevalence of ICESa2603-like ICEs in S. agalactiae from Oreochromis niloticus in Guang-
dong province was detected by PCR. The rate of ICE-carrying bacteria increased from 78.8% in isolates from 2014 to 2016
to 96.3% isolated from 2020 to 2021, indicating an upward trend. Conjugation experiments verified that ICE Sag1535 can
excise itself from the genome, cyclize, self-transfer, and inserted into the recipient bacterium, Sag158. The attL, attR, attB, and
oriT sites were characterized. After ICE integration into Sag158, the IS30 element was detected between the TSPSI and attR
sites. ICESag1535, identified in this study, is a complete ICE element with self-excision and cyclization activities and cross-
species conjugal transfer capability. The variable region contains a variety of complex transposons, leading to the formation of
multiple horizontal transfer mechanisms and increasing prevalence diversity. Due to the recruitment specialty of IS elements,
the variable region of ICEs exhibits open-ended evolution, conferring the host microbe with increased potential for pathogeni-
city and dissemination. This research holds significant implications for microbial safety, biological evolution, and genetic
resources research.

Key words: Oreochromis niloticus; Streptococcus agalactiae; ICE_Sag1535_mutT; Conj Tn5252 conjugation modules; triplet of
site-specific serine integrases; horizontal genes transfer; spread across species; open-ended evolution
Corresponding author: ZHAO Lijuan. E-mail: zhaolijuan4234@163.com
Funding projects: Guangdong Provincial Special Fund For Science and Technology Innovation Strategy and Rural Revitaliza-
tion Strategy in 2021 (2021S0082); the Second Batch of Social Welfare and Basic Research Fund of Zhongshan in 2022
(2022B2003); Guangdong Provincial Special Fund for Modern Agriculture Industry Technology Innovation Teams
(2019KJ141)

中国水产学会主办 sponsored by China Society of Fisheries https://www.china-fishery.cn
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