A Connection between Natural Transformation and Gene Transduction in Bacteria
Gene transfer agents (GTAs) resemble double-stranded DNA bacteriophages but package only random segments of the producing cell's genome. Brimacombe et al. (p. 2653–2663) discovered that genes encoding homologues of ComEC and ComF, which are defining proteins in natural transformation systems, are required for Rhodobacter capsulatus cells to receive genetic markers carried by particles of the R. capsulatus GTA (RcGTA). They further show that these genes are needed for DNA transport to the cytoplasm and are coinduced with RcGTA itself. These results indicate that the RcGTA-mediated genetic exchange process is a blend of transduction and natural transformation.
An Antirepressor That Binds Cobalamin
PpaA is an antirepressor that regulates expression of photosynthesis genes in Rhodobacter sphaeroides. Vermeulen and Bauer (p. 2694–2703) show that PpaA's antirepressor activity requires bound hydroxycobalamin. The authors show that cobalamin binding is a common feature among six PpaA homologues present in species that synthesize a photosystem. Most PpaA homologues specifically bind hydroxycobalamin, but some are capable of binding other forms of cobalamin that have a bulkier upper ligand. Hydroxycobalamin is a known product of photohydrolysis or redox reactions, suggesting that some species utilize binding of hydroxycobalamin by PpaA to sense light or redox conditions.
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