Restart of DNA replication in Gram-positive bacteria: functional characterisation of the Bacillus subtilis PriA initiator

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Oxford University Press

RESUMO

The PriA protein was identified in Escherichia coli as a factor involved in the replication of extrachromosomal elements such as bacteriophage φX174 and plasmid pBR322. Recent data show that PriA plays an important role in chromosomal replication, by promoting reassembly of the replication machinery during reinitiation of inactivated forks. A gene encoding a product 32% identical to the E.coli PriA protein has been identified in Bacillus subtilis. To characterise this protein, designated PriABs, we constructed priABs mutants. These mutants are poorly viable, filamentous and sensitive to rich medium and UV irradiation. Replication of pAMβ1-type plasmids, which is initiated through the formation of a D-loop structure, and the activity of the primosome assembly site ssiA of plasmid pAMβ1 are strongly affected in the mutants. The purified PriABs protein binds preferentially to the active strand of ssiA, even in the presence of B.subtilis SSB protein (SSBBs). PriABs also binds stably and specifically to an artificial D-loop structure in vitro. These data show that PriABs recognises two specific substrates, ssiA and D-loops, and suggest that it triggers primosome assembly on them. PriABs also displays a single-stranded DNA-dependent ATPase activity, which is reduced in the presence of SSBBs, unless the ssiA sequence is present on the ssDNA substrate. Finally, PriABs is shown to be an active helicase. Altogether, these results demonstrate a clear functional identity between PriAEc and PriABs. However, PriABs does not complement an E.coli priA null mutant strain. This host specificity may be due to the divergence between the proteins composing the E.coli and B.subtilis PriA-dependent primosomes.

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