Horizontal gene transfer (HGT) is the mechanism throughwhich genomic islands move between bacteria. The three major HGT mechanisms areconjugation (gene exchange through plasmids), transformation (gene exchangethrough extracellular DNA) and transduction (virus or phages transferring thegene).Transformation:The termtransformation refers to the process of HGT in which DNA uptake from theenvironment occurs (6,7). The ability to uptake exogenous DNA is known as”natural competence”. In bacteria, natural competence is a complex process thatrequires the expression of genes involved in the assembly of type IV pili andtype II secretion systems (6,8).
Expression of these sets of genes (about 40genes in Bacillus subtilis) depends on specific physiological and environmentalcues such as high cell density and limited nutrient availability . Certainbacterial species, such as Neisseria gonorrhoeae and Haemophilus infuenzae, arealways competent to accept DNA, whereas others, such as Bacillus subtilis andStreptococcus pneumoniae, become competent upon reaching a certainphysiological stage in their life cycle(1,7).A largeand versatile family of Type4 Secretion System-dependent transport systemscontains conjugation systems 32-33. These are encoded by multiple genes whichare organized into a single operon. according to the organization of geneticdeterminants, shared homologues and evolutionary relationships,type 4 secretionsystem can be classified into several types 34.
T4SSs have a role in thehorizontal transfer of a wide variety of genomic islands in a broad spectrum ofbacteria, including Haemophilus spp., Pseudomonas spp., and S. enterica serovarTyphi. 34-36.
Genes necessary for formation of a conjugative pilus and otherproteins necessary for the island transfer are encoded by the T4SS of ICE Hin1056 35. A better explanation of how GEIs can propagate and efficiently enablebacterial population to adapt to rapidly changing environments can be providedby the presence of a highly evolved and efficient conjugation system formobilizin ggenomic islands.Thecomponents involved in DNA-uptake are not the same for gram-positive andgram-negative bacteria due to the difference in cell wall structure.
Ingram-positive bacteria, retraction of a pseudopilus opens a cell wall hole thatallows DNA to diffuse from the surface. In gram-negative bacteria, due to thepresence of an extra membrane, DNA uptake requires the presence of a morecomplex channel, mainly formed by secretins (PilQ). In contrast to DNA uptake,DNA translocation across the cell membrane is similar in gram-negative andgram-positive bacteria. In both groups, homologues of the ComEC channelproteins mediate the transport of the DNA to the cytoplasm. During thisprocess, one strand of the incoming DNA is degraded by nucleases, and theremaining single-stranded DNA is bound by proteins that protect it from degradation.Incorporation into the chromosome can be catalyzed by the mechanisms ofhomologus recombination if sufficient sequence identity exists(2,6).