Expression, purification and biochemical characterization of Listeria monocytogenes single stranded DNA binding protein 1.

Single-stranded DNA binding proteins play an important role in DNA
metabolic processes including replication, recombination, and repair. Here, we report
the identification and biochemical characterization of the SSB1 protein from the
foodborne pathogen Listeria monocytogenes. The L. monocytogenes SSB1 share
33% identity and 50.5% similarity with the prototype E. coli SSB protein. The
electrophoretic mobility shift assay revealed that the purified L. monocytogenes
SSB1 protein binds to single stranded DNA, including the M13 circular single
stranded DNA and oligonucleotide, with high affinity. The plasmid based strand
transfer activity showed that, in the absence of the SSB protein, L. monocytogenes
RecA fails to catalyze the reaction whereas, the E. coli RecA protein has shown
nicked DNA formation. Interestingly the addition of SSB1 protein stimulates both L.
monocytogenes and E. coli RecA strand transfer activities however, it is sensitive to
the order of addition of SSB1 protein. L. monocytogenes RecA fails to catalyze the
reaction when SSB1 is added prior to RecA; nevertheless, it readily catalyzes the
reaction when added after the RecA filament formation. These results suggest that
the interaction among of gene product between RecA and SSB1 is required to
promote optimum strand exchange activities. Altogether, these studies provide the
first functional characterization of the L. monocytogenes SSB1 protein and give
insights into DNA repair and recombination processes in the gram-positive
foodborne pathogen L. monocytogenes.