A septal chromosome segregator protein evolved into a conjugative DNA-translocator protein

Abstract

Streptomycetes, Gram-positive soil bacteria well known for the production of antibiotics feature a unique conjugative DNA transfer system. In contrast to classical conjugation which is characterized by the secretion of a pilot protein covalently linked to a single-stranded DNA molecule, in Streptomyces a double-stranded DNA molecule is translocated during conjugative transfer. This transfer involves a single plasmid encoded protein, TraB. A detailed biochemical and biophysical characterization of TraB, revealed a close relationship to FtsK, mediating chromosome segregation during bacterial cell division. TraB translocates plasmid DNA by recognizing 8-bp direct repeats located in a specific plasmid region clt. Similar sequences accidentally also occur on chromosomes and have been shown to be bound by TraB. We suggest that TraB mobilizes chromosomal genes by the interaction with these chromosomal clt-like sequences not relying on the integration of the conjugative plasmid into the chromosome. Elucidation of the molecular function of TraB highlights how nature is able to develop new machineries from other cellular processes. Whereas DNA transfer by classical conjugation is done by specialized type IV protein secretion systems, for the conjugative DNA transfer in Streptomyces a chromosome segregation system was rebuild.

Achnowledgments

The research was supported by the DFG (SFB766) and by a DAAD fellowship to E.S.

Figures and Tables

Figure 1 Model of conjugative DNA translocation in mycelial streptomycetes. After establishing contact between donor (gray) and recipient mycelium (white, dashed lines indicate neighboring mycelial compartments) and partial fusion of the hyphal tips, TraB hexamers (blue) assemble at clt by specifically recognizing 8-bp TRS repeats. The TraB hexamers form pore structures to the recipient and direct the translocation of a double-stranded plasmid molecule. DNA transfer is energized by ATP hydrolysis. Up to now it is unclear (?) whether the circular plasmid DNA has to be processed into a linear molecule and which enzymes might be involved. For further description see text.

Table 1 PAT SCAN analysis* of the S. coelicolor genome for the presence of clt_pSVH1-like chromosomal sequences (clc)*