Bacterial Primosome


DNA (deoxyribonucleic acid) replication requires operation of a molecular machinery which efficiently synthesises of nucleotide chains on both strands. This process requires not only the enzymes synthesising DNA (DNA polymerases) but also those providing primer RNAs (ribonucleic acid) and continuously melting the duplex DNA. The primosome refers to a protein complex capable of processive unwinding of duplex DNA and primer RNA synthesis on the lagging strand at a replication fork. The prepriming proteins, DNA helicase and primase are sequentially assembled on the template DNA to generate primosome. Once assembled, it, in conjunction with DNA polymerases, facilitates DNA chain elongation. The assembly of bacterial primosome is triggered by an ‘initiator’ protein including DnaA and PriA, which recognise the site of assembly. Primosome is reassembled in replication restart process at stalled or processed replication forks, triggered by PriA. Thus, primosome constitutes an essential component for active replication fork machinery.

Key concepts:

  • Replication fork is the site of DNA replication where two replicating single‐stranded DNA separates.

  • Primer RNA is a short stretch of RNA, the 3′‐terminus of which is utilised by DNA polymerases for DNA elongation.

  • Primosome is a name given to the protein complex capable of duplex DNA unwinding and primer RNA synthesis at the replication fork.

  • oriC is the replication origin of chromosome. The initiation site of bacterial chromosomal DNA replication under a normal growth condition.

  • DnaA is the initiator protein for bacterial chromosomal replication, which binds to oriC to assemble a primosome.

  • PriA is a conserved replication factor which triggers assembly of the so‐called ϕX174‐type primosome, which is assembled at a stalled replication fork for replication restart.

  • Stalled replication fork, the replication fork the movement of which is blocked by internal and external ‘replication stress’ including DNA damages and depletion of nucleotide precursors.

  • Replication restart is a process of reassembly of primosome at a stalled replication fork to resume DNA chain elongation.

  • Recombination intermediate is the intermediate structure (e.g. D‐loop structure) of homologous recombination reaction.

  • DNA helicase is a protein capable of unwinding a duplex DNA at a replication fork by using the energy derived from the hydrolysis of nucleotides.

  • Prepriming proteins are proteins required for the stage preceding the association of the primase (an enzyme synthesising primer RNAs) during the assembly of a primosome.

Keywords: DnaA; PriA; DnaB helicase; DnaG primase; replication fork

Figure 1.

Two representative modes of primosome assembly for DNA replication in E. coli. PriA‐dependent primosome is assembled at a small single‐stranded hairpin structure, whereas DnaA‐dependent primosome is assembled at oriC DNA as well as at a hairpin containing dnaA box (A site). The two modes differ in the requirement of proteins involved in the prepriming stage. Hypothetical structures for the preprimosomes are shown and the association of DnaG primase with them results in functional primosomes. The PriA‐dependent ‘ϕX174‐type’ primosome can be assembled at a recombination intermediate or at a stalled replication fork.



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Further Reading

Marians KJ (1992) Prokaryotic DNA replication. Annual Review of Biochemistry 61: 673–719.

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Tanaka, Taku, and Masai, Hisao(Apr 2010) Bacterial Primosome. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0001048.pub2]