Abstract
Rhodobacteraceae harbor a conspicuous wealth of extrachromosomal replicons (ECRs) and therefore the exchange of genetic material via horizontal transfer has been supposed to be a major evolutionary driving force. Many plasmids in this group encode type IV secretion systems (T4SS) that are expected to mediate transfer of proteins and/or DNA into host cells, but no experimental evidence of either has yet been provided. Dinoroseobacter shibae, a species of the Roseobacter group within the Rhodobacteraceae family, contains five ECRs that are crucial for anaerobic growth, survival under starvation and the pathogenicity of this model organism. Here we tagged two syntenous but compatible RepABC-type plasmids of 191 and 126-kb size, each encoding a T4SS, with antibiotic resistance genes and demonstrated their conjugational transfer into a distantly related Roseobacter species, namely Phaeobacter inhibens. Pulsed field gel electrophoresis showed transfer of those replicons into the recipient both individually but also together documenting the efficiency of conjugation. We then studied the influence of externally added quorum sensing (QS) signals on the expression of the T4SS located on the sister plasmids. A QS deficient D. shibae null mutant (ΔluxI1) lacking synthesis of N-acyl-homoserine lactones (AHLs) was cultivated with a wide spectrum of chemically diverse long-chain AHLs. All AHLs with lengths of the acid side-chain ≥14 reverted the ΔluxI1 phenotype to wild-type. Expression of the T4SS was induced up to log2 ∼3fold above wild-type level. We hypothesize that conjugation in roseobacters is QS-controlled and that the QS system may detect a wide array of long-chain AHLs at the cell surface.
Highlights
Roseobacters are a group of Rhodobacteraceae which are widely distributed in marine habitats and can reach abundances up to 20 percent in polar regions or in the North Sea (Selje et al, 2004; Giebel et al, 2011) and during algae blooms (Wemheuer et al, 2014; Voget et al, 2015)
We investigated if the transcription of the T4SS on those two plasmids of D. shibae can be induced by chemically diverse acyl-homoserine lactones (AHLs), most of which have been shown to be produced by isolates from the Rhodobacteraceae but not necessarily by D. shibae
Our experiments demonstrate the conjugative transfer of both the 126 and the 191-kb plasmid of D. shibae into P. inhibens, suggesting that the T4SS located on them is mediating conjugation as predicted by in silico analyses
Summary
Roseobacters are a group of Rhodobacteraceae which are widely distributed in marine habitats and can reach abundances up to 20 percent in polar regions or in the North Sea (Selje et al, 2004; Giebel et al, 2011) and during algae blooms (Wemheuer et al, 2014; Voget et al, 2015). Type IV secretion systems (T4SS) were found in half of the 12 genome sequences of roseobacters available at the time (Moran et al, 2007) They are highly homologous to the archetypical VirB/VirD4 operon of Agrobacterium tumefaciens (Christie et al, 2014) and might function to transfer DNA and/or proteins into other bacteria or even eukaryotic hosts such as dinoflagellates (Wagner-Döbler et al, 2010; Petersen et al, 2013; Wang et al, 2015). D. shibae utilizes a complex communication system comprising two chromosomal luxI/luxR operons (luxI1/luxR1, luxI2/luxR2), a third synthase gene (luxI3) located on the 86-kb plasmid downstream of an autoinducer binding gene luxB (Wang et al, 2014b) and three additional orphan luxR transcriptional regulators (Wagner-Döbler et al, 2010) These QS circuits are organized in a hierarchical way (Patzelt et al, 2013). We investigated if the transcription of the T4SS on those two plasmids of D. shibae can be induced by chemically diverse AHLs, most of which have been shown to be produced by isolates from the Rhodobacteraceae but not necessarily by D. shibae
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
