UPTAKE AND PROCESSING OF DNA BY ACINETOBACTER-CALCOACETICUS - A REVIEW

In natural transformation, DNA in the form of macromolecular fragments can be translocated across the cell envelope of prokaryotic microorga nisms. During the past two decades, several, largely mutually contradi ctory, hypotheses have been forwarded to explain the molecular mechani sm and bioenergetics of this translocation process. Other biomacromole cules are translocated across the bacterial cell envelope as well, suc h as polysaccharides and proteins, the latter for instance in the proc ess of the assembly of type-IV pili. This brings up the question wheth er or not common components are involved. Here, we review analyses of DNA translocation in Acinetobacter calcoaceticus, a Gram-negative euba cterium that is able to migrate through twitching motility, and also s hows a high frequency of natural transformation. DNA uptake in this or ganism is an energy-dependent process. Upon entry into the cells, the DNA fragments are integrated into the resident chromosome when a suffi ciently large region of mutual homology is available (200 to 400 bp). However, this process is rather inefficient, and on the average 500 bp of each incoming fragment is degraded through exonuclease activity. U pon covalent attachment of a bulky protein molecule to the transformin g DNA, the DNA-translocation machinery becomes blocked in further tran slocation activity. Since A. calcoaceticus is not well suited for tran sposon mutagenesis, a random mutagenesis procedure has been developed, based on the ligation of an antibiotic-resistance marker to random fr agments of chromosomal DNA. This method was used to generate several m utants impaired in the natural transformation process. Three of these have been characterized in detail. No components, common to the transl ocation of macromolecules through the cell envelope of Acinetobacter, have been detected in this screen. (C) 1997 Elsevier Science B.V.