A widely conserved molecular switch controls quorum sensing and symbiosis island transfer in Mesorhizobium loti through expression of a novel antiactivator

Abstract

ICEMlSym(R7A) of Mesorhizobium loti is an integrative and conjugative element (ICE) that confers the ability to form a nitrogen-fixing symbiosis with Lotus species. Horizontal transfer is activated by TraR and N-acyl-homoserine lactone (AHL), which can stimulate ICE excision in 100% of cells. However, in wild-type cultures, the ICE is excised at low frequency. Here we show that QseM, a widely conserved ICE-encoded protein, is an antiactivator of TraR. Mutation of qseM resulted in TraR-dependent activation of AHL production and excision, but did not affect transcription of traR. QseM and TraR directly interacted in a bacterial two-hybrid assay in the presence of AHL. qseM expression was repressed by a DNA-binding protein QseC, which also activated qseC expression from a leaderless transcript. QseC differentially bound two adjacent operator sites, the lower affinity of which overlapped the -35 regions of the divergent qseC-qseM promoters. QseC homologues were identified on ICEs, TraR/TraM-regulated plasmids and restriction-modification cassettes, suggesting a conserved mode of regulation. Six QseC variants with distinct operators were identified that showed evidence of reassortment between mobile elements. We propose that QseC and QseM comprise a bimodal switch that restricts quorum sensing and ICEMlSym(R7A) transfer to a small proportion of cells in the population.

Assembly and transfer of tripartite integrative and conjugative genetic elements

“…Transfer occurred at a frequency of 4.65 × 10 −8 ± 7.89 × 10 −9 (SE) from WSM1271, 8.5 × 10 −9 ± 8.5 × 10 −10 from WSM2073, and 3.0 × 10 −9 ± 6.0 × 10 −10 from WSM2075 donors. In comparison, the rate of transfer of ICEMlSym R7A from M. loti R7A is ∼1 × 10 −7 , but increases to ∼1 × 10 −4 in derepressed strains (15)(16)(17). Exconjugants from 16 independent experiments were screened by PCR for genes located on regions α, β, γ, and a region specific for the R7ANS chromosome (Table S1).…”

“…After a 7-y period, only ∼20% of L. corniculatus root nodules contained R7A; the other 80% of nodules contained a diverse mix of native mesorhizobia that had acquired symbiosis genes from R7A (11). Molecular analyses revealed these new symbionts had acquired a 502-kb symbiosis ICE, subsequently named ICEMlSym R7A (12)(13)(14).The mechanism and regulation of ICEMlSym R7A integration and excision have been investigated in detail (12,(15)(16)(17)(18). Integration of ICEMlSym R7A is catalyzed by the integrase IntS, which recombines the attachment (att) site attP located on the circularized ICEMlSym R7A with the attB site located at the 3′ end of the sole phetRNA gene present in Mesorhizobium genomes.…”

“…The mechanism and regulation of ICEMlSym R7A integration and excision have been investigated in detail (12,(15)(16)(17)(18). Integration of ICEMlSym R7A is catalyzed by the integrase IntS, which recombines the attachment (att) site attP located on the circularized ICEMlSym R7A with the attB site located at the 3′ end of the sole phetRNA gene present in Mesorhizobium genomes.…”

Plant nodulation inducers enhance horizontal gene transfer of Azorhizobium caulinodans symbiosis island

“…The ICEMlSym R7A is inserted downstream of a phe-tRNA gene in the M. loti chromosome and can be transferred to nonsymbiotic mesorhizobia both in the laboratory and in the field, converting these strains into symbionts of L. corniculatus (12,13). The transfer of ICEMlSym R7A is mediated by quorum sensing and is controlled by a complex multipartite regulatory system involving an excisionase and an excision activator produced via a programmed ribosomal frameshift (14)(15)(16).…”

Life History Analysis of Integrative and Conjugative Element Activation in Growing Microcolonies of Pseudomonas

“…Furthermore, it has been shown in Escherichia coli that the subcellular localization of proteins (27) and protein aggregates can differ among individuals in a clonal population as a result of cell age (28,29). Also, specific signaling effects at high cell density could possibly influence the appearance of TC cells (30,31), although previous experiments showed no effect of culture exposure to UV or toxic chemicals or of high and low cell density on the appearance of the excised ICEclc form (32). The major question underlying the current work was thus to study whether initiation of the ICEclc TC state is correlated in any way to distinguishable cell fates (e.g., cell age, cell lysis).…”