Plasmids are natural vectors for gene transfer. In Gram-negative
bacteria, plasmid DNA replication is triggered when monomers of an initiator
protein (Rep) bind to direct repeats at the origin sequence. Rep dimers, which
are inactive as initiators, bind to an inverse repeat operator, repressing
transcription of the rep gene. Rep proteins are composed of N-terminal
dimerization and C-terminal DNA-binding domains. Activation of Rep is coupled
to dimer dissociation, converting the dimerization domain into a second
origin-binding module. Although the structure of the monomeric F plasmid
initiator (mRepE) has been determined, the molecular nature of Rep activation
remains unknown. Here we report the crystal structure of the dimeric N-terminal
domain of the pPS10 plasmid initiator (dRepA). dRepA has a winged-helix fold,
as does its homologous domain in mRepE. However, dimerization transforms an
interdomain loop and -strand (monomeric RepE) into an -helix
(dimeric RepA). dRepA resemble the C terminus of eukaryotic and archaeal Cdc6,
giving clues to the phylogeny of DNA replication initiators.
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-strand (monomeric RepE) into an 
-helix
(dimeric RepA). dRepA resemble the C terminus of eukaryotic and archaeal Cdc6,
giving clues to the phylogeny of DNA replication initiators.
