Integrative and Conjugative Element ICETh1 Functions as a Pangenomic DNA Capture Module in Thermus thermophilus

Transjugation is an unconventional conjugation mechanism in Thermus thermophilus (Tth) that involves the active participation of both mating partners, encompassing a DNA secretion system (DSS) in the donor and an active natural competence apparatus (NCA) in the recipient cells. DSS is encoded within an integrative and conjugative element (ICETh1) in the strain Tth HB27, whereas the NCA is constitutively expressed in both mates. Previous experiments suggested the presence of multiple origins of transfer along the genome, which could generate genomic mosaicity among the progeny. Here, we designed transjugation experiments between two closely related strains of Tth with highly syntenic genomes, containing enough single nucleotide polymorphisms to allow precise parenthood analysis. Individual clones from the progeny were sequenced, revealing their origin as derivatives of our ICETh1-containing intended "donor" strain (HB27), which had acquired separate fragments from the genome of the ICETh1-free HB8 cells, which are our intended recipient. Due to the bidirectional nature of transjugation, only assays employing competence-defective HB27 derivatives as donors allowed the recovery of HB8-derived progeny. These results show a preference for a retrotransfer mechanism in transjugation in ICETh1-bearing strains, supporting an inter-strain gene-capture function for ICETh1. This function could benefit the donor-capable host by facilitating the acquisition of adaptive traits from external sources, ultimately increasing the open pangenome of Thermus, maximizing the potential repertoire of physiological and phenotypical traits related to adaptation and speciation.