Abortive transduction is the process of introducing a linear fragment of DNA from one bacterial cell into another using a bacteriophage vector. The size of the DNA fragment is determined by how much DNA the bacteriophage will package into a phage capsid, which is about 44.
Once injected into a recipient cell, the transduced DNA fragment has three possible fates:
- it can be broken down;
- it can recombine with the recipient chromosome or plasmid, resulting in a stable change in bacterial genotype (complete transduction); or
- it can establish itself as a non-replicating genetic element that is only separated into one of the two daughter cells with each division (abortive transduction).
Establishing an abortive transducing fragment may involve protein-mediated circularization of the incoming linear fragment.
Abortive transduction was first used in the 1950s by (among others) B.A.D. Stocker, Lederberg and H. Ozeki. Stocker’s transduction analyzes of motility mutants of Salmonella typhimurium using P22 were particularly informative.
Motile cells embedded in semi-solid agar can swim away from a growing colony and continue to multiply, forming a large circular swarm of cells, but an immobile mutant strain (e.g., without flagella) will multiply in place to form a small one circular colony.
A suitable abortively transduced wild-type DNA can complement the motility mutation and enable the previously non-motile cell to swim. During swimming, however, immobile daughter cells are created that remain in place and continue to multiply there. This results in a compact colony (offspring of the first daughter cell) with a trace of cells emanating from it (later offspring of the abortively transduced swim cell).
Nutritional markers (for example, mutations that negate the ability to synthesize an amino acid) can also be abortively transduced, resulting in very small colonies on minimal media that lack the required nutrient.
Such markers were used to study the process of abortive transduction using P1 in Escherichia coli and P22 in S. typhimurium. In fact, abortive transduction is more common than full transduction – up to 90% of all transduction fragments introduced into cells establish themselves as abortive transductants, while about 2% form complete transductants.
The physical nature of abortive transduction has been studied by Sandri and Berger, von Schmeiger, and others. One method uses the infection of unlabeled cells with phage grown on bacteria with labeled DNA. The fate of the labeled DNA can be followed by density separation for heavy non-radioactive isotopes such as 15N.
Only about 10-15% of the label in the fragments is physically associated with the unlabeled chromosome (either by recombination or by nucleotide recycling). The remaining marking is not degraded and can be obtained quantitatively for at least 5 hours after it has been introduced.
This persistent state is consistent with the genetic observation that DNA can replenish defective chromosomal genes for many generations. Complete transduction occurs within the first hour after introduction.
The physical protection of the abortive fragments from host nucleases appears to result from the protein associated with the DNA. Abortive transducing DNA labeled with heavy isotopes exhibited an accelerated sedimentation rate consistent with a super-coiled circle when reisolated from recipient cells; the sedimentation rate was normalized again by protease treatment.
In the P22 system, a specific phage protein was involved in the protection process: P22 gene 16 mutants produce fewer abortive transductants, but a normal number of complete transductants. It is believed that the protein is packaged with the DNA in the capsid and injected with the transducing fragment.
The biological effects of this process are difficult to assess. Their abundance in nature is unknown. It could, in principle, allow the cell to escape from a stressful state for so long that it receives a new mutation adaptation or finds a new environment without leaving a permanent genetic record of the event.