Light Cast to Broken DNA, Its Repair, and Consequences
PCNA is an ancient DNA-proliferator. Referred to as “DNA sliding clamp”, the proliferating cell nuclear antigen (PCNA) is present in the a-nucleate Haloarchaeal (phylum Euryarchaeota) genomes . How it appeared there first is not clear at all. Halobacteriales (Haloarchaea) received and accepted laterally transferred genes even from eubacteria. These genes underwent duplications and lateral transfers within the 29 genera of the halobacterial clade. Another enzyme, the basic leucyl-tRNA synthetase is also an ancient acquisition of Euryarchaeota origin. Thus, at least one cluster of this enzyme was received from outside source and it did not evolve dependently of the halobacterial tRNA . Crenarchaeota possess another distinct PCNA. The chromatine-bond PCNA in the form of homotrimeric rings encircles DNA double helix, serving as platform for other recruited proteins, especially the auxiliary factor for DNA polymerase (DNA polymerase δ processivity factor). The replisomes of replicons form clusters known as replication factories, or foci. These foci contain the DNA polymerases, their anchor, the PCNA, replication factor C, replication protein A, DNA ligases, and cyclin A. PCNA acts in cell cycle control, and in the processes of DNA elongation, recombination, methylation and damage repair. Viral DNA replicating in host cell cytoplasm (vaccinia virus) or nucleus (polyoma virus) also forms large replication factories (“virus factories”). The PCNA is a homotrimer DNA-sliding clamp, that interacts with (it recruits) the DNA ligase, that also encircles the DNA nick (the DNA-binding domain; DNA replication foci) [1067–1071].