Gene transfer agent

Among the GTAs mentioned by the article, RcGTA and DsGTA are now in the family Rhodogtaviriformidae, BaGTA in Bartogtaviriformidae, and VSH-1 in Brachygtaviriformidae.

The first GTA system was discovered in 1974, when mixed cultures of Rhodobacter capsulatus strains produced a high frequency of cells with new combinations of genes.

[6] The factor responsible was distinct from known gene-transfer mechanisms in being independent of cell contact, insensitive to deoxyribonuclease, and not associated with phage production.

Where studies of sequence divergence have been done (dN/dS analysis), they indicate that the genes are being maintained by natural selection for protein function (i.e. defective versions are being eliminated).

[10]   One alternative explanation is that GTA genes persist because GTAs are genetic parasites that spread infectiously to new cells.

The structural genes are first transcribed and translated, and the proteins assembled into empty heads and unattached tails.

The two best studied GTAs (RcGTA and BaGTA) randomly package all of the DNA in the cell, with no overrepresentation of GTA-encoding genes.

[14] Second, particles must encounter and attach to suitable recipient cells, usually members of the same or a closely related species.

Like phages, GTAs attach to specific protein or carbohydrate structures on the recipient cell surface before injecting their DNA.

Regulation of GTA production and transduction has been best studied in R. capsulatus, where a quorum-sensing system and a CtrA-phosphorelay control expression of not only the main RcGTA gene cluster, but also a holin/endolysin cell lysis system, particle head spikes, an attachment protein (possibly tail fibers), and the capsule and DNA processing genes needed for RcGTA recipient function.

RcGTA-like clusters are found in a large subclade of the alphaproteobacteria, although the genes also appear to be frequently lost by deletion.

[8] D. shibae, like R. capsulatus, is a member of the Order Rhodobacterales, and its GTA shares a common ancestor and many features with RcGTA, including gene organization, packaging of short DNA fragments (4.2 kb) and regulation by quorum sensing and a CtrA phosphorelay.

Bartonella species are members of the Alphaproteobacteria like R. capsulatus and D. shibae, but BaGTA is not related to RcGTA and DsGTA.

[13] Brachyspira is a genus of spirochete; several species have been shown to carry homologous GTA gene clusters.

Comparison of typical phage (bacteriophage) infection and transduction ( A ) with typical GTA (gene transfer agent) production and transduction ( B ).
Schematic drawing of GTA assays.
Methods for detecting gene transfer agents. ( A ) Method used by Marrs in 1974. ( B ) Cell-free extract method.
Schematic diagrams of typical prophage and GTA gene clusters.
Typical prophage and GTA gene clusters.
Schematic diagram of phylogenetic relationships between known bacterial gene transfer agents.
Schematic diagram of the evolutionary forces that act on bacterial gene transfer agent and the cells that produce it.
The evolutionary forces that act on bacterial gene transfer agent and the cells that produce it.
Schematic diagram showing steps in the production of bacterial gene transfer agents.
Typical steps in the production of bacterial gene transfer agents. ( 1 ) Transcription and translation of the GTA genes. ( 2 ) Assembly of GTA structural proteins into empty heads and unattached tails. ( 3 ) Packaging of 'headful' segments of DNA into heads and attachment of tails. ( 4 ) Lysis of the cell.
Schematic diagram of genetic transduction by bacterial gene transfer agents
Genetic transduction by bacterial gene transfer agents. ( 1 ) GTA particles encounter a suitable recipient cell. ( 2 ) Particles attach to cell and inject their DNA, and cellular proteins translocate the DNA across the inner membrane. ( 3 ) DNA is degraded if it cannot recombine with the recipient genome. ( 4 ) DNA with similar sequences to the recipient genome undergoes recombination.
Regulation diagram for RcGTA, the Rhodobacter capsulatus gene transfer agent