<i>Salmonella</i> Genomic Island 3 Is an Integrative and Conjugative Element and Contributes to Copper and Arsenic Tolerance of <i>Salmonella enterica</i>

Abstract

<i>Salmonella</i> genomic island 3 (SGI3) was first described as a chromosomal island in <i>Salmonella</i> 4,[5],12:i:-, a monophasic variant of <i>Salmonella enterica</i> subsp. <i>enterica</i> serovar Typhimurium. The SGI3 DNA sequence detected from <i>Salmonella</i> 4,[5],12:i:- isolated in Japan was identical to that of a previously reported one across entire length of 81 kb. SGI3 consists of 86 open reading frames, including a copper homeostasis and silver resistance island (CHASRI) and an arsenic tolerance operon, in addition to genes related to conjugative transfer and DNA replication or partitioning, suggesting that the island is a mobile genetic element. We successfully selected transconjugants that acquired SGI3 after filter-mating experiments using the <i>S. enterica</i> serovars Typhimurium, Heidelberg, Hadar, Newport, Cerro, and Thompson as recipients. Southern blot analysis using I-CeuI-digested genomic DNA demonstrated that SGI3 was integrated into a chromosomal fragment of the transconjugants. PCR and sequencing analysis demonstrated that SGI3 was inserted into the 3' end of the tRNA genes <i>pheV</i> or <i>pheR</i> The length of the target site was 52 or 55 bp, and a 55-bp <i>attI</i> sequence indicating generation of the circular form of SGI3 was also detected. The transconjugants had a higher MIC against CuSO₄ compared to the recipient strains under anaerobic conditions. Tolerance was defined by the <i>cus</i> gene cluster in the CHASRI. The transconjugants also had distinctly higher MICs against Na₂HAsO₄ compared to recipient strains under aerobic conditions. These findings clearly demonstrate that SGI3 is an integrative and conjugative element and contributes to the copper and arsenic tolerance of <i>S. enterica</i>.