[P004] Poster

1:00 PM - 1:50 PM JST(4:00 AM - 4:50 AM UTC)

[P02-220] Structure and features of the integrative and conjugative element carrying the gene for degradation of γ-hexachlorocyclohexane

○Tomonari Mizukami¹, Shunri Oka¹, Eitaro Miyazaki¹, Mei Yamauchi¹, Kouhei Kishida¹, Yoshiyuki Ohtsubo¹, Yuji Nagata¹ (1. Graduate School of Life Sciences, Tohoku University (Japan))

Keywords:

Persistent organic pollutants (POPs),horizontal gene transfer,integrative and conjugative element (ICE),Organochlorine insecticides

Environmental pollution caused by persistent organic pollutants (POPs) remains a serious problem. We are investigating γ-hexachlorocyclohexane (γ-HCH)-degrading bacteria, with one of our aims being their application in bioremediation. Bacteria capable of assimilating anthropogenic organochlorine insecticides, such as γ-HCH, also serve as excellent models for elucidating the mechanisms of bacterial adaptation and evolution in the environment. Previous studies have strongly suggested that γ-HCH-degrading bacteria have independently emerged in various parts of the world through the acquisition of specific lin genes required for γ-HCH degradation by sphingomonad bacteria that are widely distributed in the environment. However, the environmental dynamics of these lin genes remain unclear. In this study, we identified a novel integrative and conjugative element (ICE), designated ICE_UT26, carrying one of the lin genes, linB, which encodes a haloalkane dehalogenase, in the archetypal γ-HCH-degrading strain, Sphingobium japonicum UT26. This is the first report of a lin gene being located on a mobile genetic element other than sphingomonad-specific plasmids, and no IS6100 was detected in its flanking regions. PCR-based detection of circular intermediates demonstrated that ICE_UT26 can excise from the chromosome and form a circular intermediate, a hallmark of ICE behavior. Sequence analysis of the excision junction further suggested that ICEUT26 recognizes a core attachment sequence within a tRNA-Arg gene. ICE_UT26 was transferred not only to Sphingomonas paucimobilis IAM12578, which is closely related to the UT26 strain, but also to more distantly related strains, including Burkholderia multivorans ATCC 17616 and Pseudomonas putida KT2440. However, transfer frequencies differed markedly among recipients and were highest in IAM12578, suggesting that although ICE_UT26 has a relatively broad host range, transfer efficiency is influenced by host phylogeny. In all recipient strains examined, the tRNA-Arg gene was suggested to be the integration site. It was also found that the excision frequency of ICE_UT26 increased during the stationary phase. In addition, γ-HCH affected excision in a concentration-dependent manner, with low concentrations promoting excision and high concentrations suppressing it. These results suggest that mobilization of ICE_UT26 is not constitutive, but is regulated by both the physiological state of the host and environmental conditions. Taken together, our findings identify ICE_UT26 as a novel vehicle for the horizontal transfer of lin genes. This study provides new insights into how catabolic genes for anthropogenic pollutants are disseminated and maintained in bacterial communities and advances our understanding of the evolutionary processes that shape biodegradation functions in the environment.