ubunit, and the large one modulates the activity in the heterodimeric archaeal primases. small and large subunits are also called PriS and PriL, respectively. The crystal structure of the N-terminal domain of PriL complexed with PriS of S. solfataricus primase revealed that PriL does not directly contact the active site of PriS, and therefore, the large subunit may interact with the synthesized primer, to adjust its length to a 7-14 mer. The structure of the catalytic center is similar to those of the family X DNA polymerases. 3-terminal nucleotidyl transferase activity, detected in the S. solfataricus primase, and the gap-filling and strand-displacement activities in the P. abyssi primase also support the structural similarity between PriS and the family X DNA polymerases. A unique activity, named PADT (template-dependent Polymerization Across Discontinuous Template), in the S. solfataricus PriSL complex was published very recently. activity may be involved in double-strand break repair in Archaea. The archaeal genomes also encode a sequence similar to the bacterial type DnaG primase. The DnaG homolog from the P. furiosus genome was expressed in E. coli, but the protein did not show any primer synthesis activity in vitro , and thus the archaeal DnaG-like protein may not act as a primase in Pyrococcus cells. DnaG-like protein was shown to participate in RNA degradation, as an exosome component. However, a recent paper reported that a DnaG homolog from S. solfataricus actually synthesizes primers with a 13 nucleotide length. It would be interesting to investigate if the two different primases share the primer synthesis for leading and lagging strand replication, respectively, in the Sulfolobus cells, as the authors suggested. proposed hypothesis about the evolution of PriSL and DnaG from the last universal common ancestor (LUCA) is interesting. The Sulfolobus PriSL protein was shown to interact with Mcm through Gins23. This primase-helicase interaction probably ensures the coupling of DNA unwinding and priming during the replication fork progression. , The direct interaction between PriSL and the clamp loader RFC (described below) in S. solfataricus may regulate the primer synthesis and its transfer to DNA polymerase in archaeal cells.
8. Single-stranded DNA binding protein
single-stranded DNA binding protein, which is called SSB in Bacteria and RPA in Archaea and Eukaryota, is an important factor to protect the unwound single-stranded DNA from nuclease attack, chemical modification, and other disruptions during the DNA replication and repair processes. SSB and RPA have a structurally similar domain containing a common fold, called the OB (oligonucleotide / oligosaccharide binding)-fold, although there is little amino acid sequence similarity between themmon structure suggests that the mechanism of single-stranded DNA binding is conserved in living organisms despite the lack of sequence similarity. E. coli SSB is a homotetramer of a 20 kDa peptide with one OB-fold, and the SSBs from Deinococcus radiodura...
