Dienstag, August 2, 2022
StartMicrobiologyRange and distribution of Sort VI Secretion System gene clusters in bacterial...

Range and distribution of Sort VI Secretion System gene clusters in bacterial plasmids


The T6SS is a key equipment in inter-microbial interactions to compete for niches, being usually encoded by dozens of genes, which might range relying on the taxon. So far, the T6SS has been recognized in a number of genera of seven phyla of Gram-negative micro organism, Acidobacteria, Bacteroidetes, Deferribacteres, Gemmatimonadetes, Nitrospirae, Planctomycetes, and Proteobacteria, being plentiful within the latter1,7,14,15,16. Though extensively current and various in these bacterial phyla, the T6SS has a patchy distribution, not being ubiquitous in all these bacterial genomes8,14,17,18. This implies an affiliation of the T6SS with horizontal gene switch. Certainly, the T6SS gene clusters are finally situated in genomic islands, which have the potential to be transferred, as a unit, to different cells9. Thus, different genetic components might act as carriers of the T6SS. Actually, some T6SS from Bacteroidales are related to ICEs12, and up to now, twenty-nine plasmids with T6SS have been reported3, a few of them functionally examined19,20,21.

Right here, to find out the distribution of T6SS in all bacterial plasmids, we mined 1000’s of plasmids obtainable in NCBI and recognized a whole bunch (n = 330) of T6SS-harboring plasmids, primarily in Proteobacteria. The T6SS was current in plasmids with a variety of sizes (27 kb–2.8 Mb), which might suggest totally different health prices (relying on the dimensions of the plasmid), thus imposing a restriction on the vertical and horizontal replication of those plasmids. Moreover, for micro organism that carry smaller plasmids with T6SS, they’d be extra prone to present a T6SS-associated phenotype as a result of greater copy variety of this gene set. Though current in a number of phyla and genera14, the T6SS distribution in plasmids is restricted, as solely ~ 1% of them encoded this secretion system. Some elements could contribute to this phenomenon: (i) the dissemination of the T6SS by way of plasmids, at the very least in Proteobacteria, appears to have boundaries, since micro organism with chromosomal T6SS (plentiful in Proteobacteria) could current a protection mechanism by way of T6SS towards the acquisition of recent plasmids3; (ii) carrying an additional copy of the T6SS doesn’t appear advantageous if the bacterium already has a chromosomal copy, as it’s a niche-specific system and totally different T6SSs don’t confer totally different features, relying extra on the effectors which are secreted (except these T6SSs are regulated in another way)10. Moreover, a lot of the T6SS-harboring plasmids recognized on this examine are giant (> 100 kb), which might doubtless pose boundaries to their acquisition and upkeep.

Beforehand, Abby et al. (2016) confirmed that the T6SS was extra prevalent in γ-Proteobacteria than in α- and β-Proteobacteria (genomes with and with out plasmids)14, and curiously, right here, we noticed that the plasmid T6SS prevails in α- and β-Proteobacteria. This distinction might be as a result of datasets used, since Abby et al. (2016) used solely full genomes (some with related plasmids)14, and we thought-about solely plasmids. Alternatively, plasmids with T6SS had been extra prevalent in γ-Proteobacteria when contemplating these carrying lower than 9 T6SS genes, indicating {that a} degradation course of could also be underway. Right here we additionally noticed the T6SS in two plasmids from different phyla, Acidobacteria and Gemmatimonadetes, which might point out their acquisition from a phylum during which plasmids carrying T6SS are prevalent. Curiously, most T6SS-harboring plasmids (~ 78%) had been current in bacterial genomes that didn’t encode chromosomal T6SS, indicating {that a} cell platform (plasmids on this case) could characterize the only real supply of the T6SS for some bacterial genomes.

Of the 4 T6SS varieties (T6SSi-iv), T6SSi was the one one discovered within the plasmids. Certainly, this kind can be the commonest sort of T6SS in Proteobacteria14. Contemplating T6SSi subtypes, i1 and i2 subtypes prevail within the chromosomes and plasmids of Abby et al. (2016) dataset14, whereas the i4b and i3 subtypes prevail within the plasmids, the primary (i4b) being predominant in a selected taxon, Ralstonia solanacearum (Desk S1). Once more, this distinction should be associated to the datasets used, as our dataset contained extra α-Proteobacteria and β-Proteobacteria, whereas Abby et al. (2016) had a dataset with a prevalence of γ-Proteobacteria. Curiously, regardless of the T6SSi being the canonical one, which might encode the 13 core parts of T6SS, right here we observe a large variability of this quantity within the plasmids with the T6SSi. Of be aware, there was a subcluster within the i3 subtype clade with dozens of plasmid sequences that weren’t carefully associated to any reference chromosomal sequence (e.g., NZ_CP025431.1, NZ_CP070369.1, NZ_ CP015063.1, NC_014918.1, NZ_CP006880.1, NZ_CP013589.1). Most of those sequences belonged to environmental micro organism and could possibly be evolving independently of the others i3 subtype sequences.

Contemplating the gene cargo of the analyzed plasmids, we didn’t observe in most of them a prevalence of resistance or virulence genes (disregarding the T6SS). The few plasmids recognized carrying resistance and virulence genes had been primarily from micro organism remoted from human or animal hosts. Certainly, medical T6SS-positive micro organism had been noticed to have a better resistance and frequency of virulence genes17. Among the many T6SS effectors recognized within the plasmids, most of them play a task in virulence, but in addition in bacterial competitors underneath stress situations, similar to ModA, which supplies a progress benefit underneath anaerobiosis22, and TseZ, which is a zinc-scavenging protein underneath oxidative stress situations23. Thus, except these T6SSs play a virulence position of their host area of interest, these plasmids could be extra associated to ecological roles. Even as a result of a few of them additionally encode secondary metabolites associated to survival and safety. The ecological gene cargos of the T6SS-harboring plasmids recognized right here distinction with virulent T6SS-harboring plasmids from medical micro organism, similar to Cronobacter spp. and Campylobacter jejuni24,25.

Though 120/330 of the T6SS-carrying plasmids had been predicted to be conjugative or mobilizable, their median dimension (~ 435 kb) would characterize a pure restriction on transmission. Thus, the mobility marker genes of those plasmids could possibly be related to different cell components, similar to genomic islands. Actually, GIs have been predicted on a whole bunch of T6SS-harboring plasmids. Nevertheless, solely three GIs encompassed the T6SS gene clusters, suggesting that mobilization of the T6SS to plasmids could have taken place a very long time in the past and there are not any extra traces (based mostly on the tactic used for detection), or there are different mechanisms of mobilization of the T6SS to plasmids.

Lastly, in dozens of those T6SS-harboring plasmids we recognized genes primarily related to chromosomes, similar to rRNA, and this, added to the truth that most of them are megabases in dimension (~ 60% of the Mb dimension group had rRNA genes vs ~ 4% of the Kb dimension group), raised the query of whether or not they had been in actual fact plasmids or one other sort of replicon. Just lately, Schmartz et al. (2022)26 analyzed putative plasmids by way of the presence of ribosomal genes to determine mislabelled sequences. Thus, some sequences initially thought-about in our examine had been filtered. Even so, some sequences containing ribosomal genes remained in our evaluation, as they produce other components that characterize them as plasmids (for instance, the presence of the rep gene). Actually, among the genera recognized right here had been related to secondary important replicons (secondary chromosomes and chromids), similar to Burkholderia, Cupriavidus, Ensifer/Sinorhizobium, Pantoea, Ralstonia, Rhizobium, Vibrio27,28. It may be speculated that this might clarify the big variety of components with T6SS predicted as non-mobilizable, since chromids, for instance, are inclined to lose the flexibility to transmit horizontally, thus turning into „caught“ to a specific genome. Actually, non-mobilizable megaplasmids can bear processes to turn into chromids28. Thus, the identification of those replicons of those organisms continues to be underneath debate (megaplasmid, chromid, or secondary chromosome).

Due to this fact, our findings don’t totally assist the speculation that T6SS unfold inside micro organism was plasmid-mediated, as occurred with T7SS in Mycobacteriaceae13. Moreover, most T6SSs from the chromosomal and plasmid compartments don’t appear to evolve independently, as noticed within the phylogeny, reinforcing that the T6SS areas could also be underneath fixed gene circulation. Even so, the proof gathered right here factors to the involvement of cell platforms within the unfold of the T6SS inside micro organism.

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