Acquisition of foreign DNA by natural transformation in Acinetobacter baylyi: quantitative estimates and molecular characterization of the introgression process

Abstract

Very little is known about introgression in bacteria. Introgression is the process where the genes of one species infiltrate the gene pool of another organism by subsequent backcross transformations of a hybrid with one of its parents. After the initial acquisition of foreign DNA, DNA from the newly made transformants is used as donor DNA in backcross transformations with the recipient. DNA is released to the environment after decomposing of dead cells, disrupting of cells or through excretion from living organisms. The extracellular DNA can be degraded after release, thus fragmented DNA can be taken up by bacteria.

One previous unpublished study investigated the effect of introgression of foreign unselected DNA. The donor DNA in this study was of high molecular weight (20 to 30 kilo bases (Kb)) and it was suggested that introgression in backcross transformation could be a mechanism by which unselected DNA was eliminated from the genome.

I wanted to study the effect of introgression when the foreign donor DNA was of low molecular weight (1000 to 4000 base pairs (bp)). We wanted to determine how fragmentation affects the speed at which unselected DNA from Acinetobacter sp. strain 16.4 was eliminated from the genome of Acinetobacter baylyi strain BD413 during the introgression process.

I have developed a method for optimal fragmentation of DNA to the desired size for this study. The DNA was fragmented by sonication, which gave an effective, gradual reduction in the fragment size of DNA. The size of the sonicated DNA was checked on an agarose gel and I found out that a gel fraction between 1000 to 4000 bp was the desired size for fragmented DNA. The DNA was extracted from the gel piece and used as low molecular weight donor DNA.

This method can be used to determine the effect of introgression when the foreign donor DNA is of low molecular weight (1000 to 4000 base pairs bp) and to get a better understanding for natural fragmentation of extracellular DNA in the environment.