Unusual Integrase Gene Expression on the clc Genomic Island in Pseudomonas sp. Strain B13

Abstract

An unusual type of gene expression from an integrase promoter was found in cultures of the bacterium Pseudomonas sp. strain B13. The promoter controls expression of the intB13 integrase gene, which is present near the right end of a 105-kb conjugative genomic island (the clc element) encoding catabolism of aromatic compounds. The enzymatic activity of integrase IntB13 is essential for site-specific integration of the clc element into the bacterial host's chromosome. By creating transcription fusions between the intB13 promoter and the gfp gene, we showed that integrase expression in strain B13 was inducible under stationary-phase conditions but, strangely, occurred in only a small proportion of individual bacterial cells rather than equally in the whole population. Integrase expression was significantly stimulated by growing cultures on 3-chlorobenzoate. High cell density, heat shock, osmotic shock, UV irradiation, and treatment with alcohol did not result in measurable integrase expression. The occurrence of the excised form of the clc element and an increase in the rates of clc element transfer in conjugation experiments correlated with the observed induction of the intB13-gfp fusion in stationary phase and in the presence of 3-chlorobenzoate. This suggested that activation of the intB13 promoter is the first step in stimulation of clc transfer. To our knowledge, this is the first report of a chlorinated compound's stimulating horizontal transfer of the genes encoding its very metabolism.

Horizontal Transfer of the High-Pathogenicity Island ofYersinia pseudotuberculosis

“…and Tn916 in Enterococcus faecalis is increased in the presence of tetracycline (6,10,26). Similarly, the presence of 3-chlorobenzoate in the culture medium enhances the transfer of the clc genomic island of Pseudomonas spp., which carries genes involved in 3-chlorobenzoate metabolism (34,40). Since the HPI encodes the yersiniabactin system, which provides a growth advantage under iron-limiting conditions (15,16, 32), we tested the impact of iron deprivation on Tf HPI .…”

The Accessory Genome of Pseudomonas aeruginosa

“…For example, though the spontaneous excision rates for the mobilizable ICEs pKL102, clc, and PAPI-1 have been determined in vitro (110,177,194,195), how these laboratory rates compare to mobilization rates in the natural environment has yet to be determined. Likewise, the in vivo growth conditions affecting ICE transfer from one bacterium to another through GI-type T4SSs are unknown, though in vitro findings suggest that ICE mobilization occurs at late growth stages and may be affected by cyclic AMP levels (35).…”

“…Though the functions of many of the genes within this highly conserved backbone have yet to be demonstrated experimentally, they are predicted to mediate excision, self-transfer to a new host, and reintegration. Currently, the mechanism of excision from the P. aeruginosa chromosome is not understood, although integrases appear to be necessary for this activity (29,124,177,180,194). Once excised, ICEs circularize to reform an attP site and restore the attB site on the P. aeruginosa chromosome (30,177,182).…”

Highly variable individual donor cell fates characterize robust horizontal gene transfer of an integrative and conjugative element

“…1C) (25). Initiation of transfer competence occurs in stationary phase, when the bacteria in culture run out of nutrients and stop dividing, but affects only 3-5% of the cells in a clonal population harboring WT ICEclc (17,19,26). This proportion can increase to 50% or more in strains with a deletion of the ICEclc gene mfsR (major facilitator superfamily regulator) (27).…”

“…Conjugation frequencies of ICEs vary widely from approximately 1 × 10 −2 to 1 × 10 −7 per donor cell, which is considered to be the outcome of an infrequent bistable switch triggering activation of the ICE (16,17). Environmental cues eliciting the "SOS response" (18) or growth on specific carbon substrates (19) can enhance ICE transfer rates, likely by influencing the frequency of the bistable switch.…”