during nucleotide excision repair in e. coli which proteins identify the damaged dna
Nucleotide excision repair The process of nucleotide excision repair is controlled in Escherichia coli by the UvrABC endonuclease enzyme complex, which consists of four Uvr proteins: UvrA, UvrB, UvrC, and DNA helicase II (sometimes also known as UvrD in this complex). First, a UvrA-UvrB complex scans the DNA, with the UvrA subunit recognizing distortions in the helix, caused for example by pyrimidine dimers. When the complex recognizes such a distortion, the UvrA subunit leaves and an UvrC protein comes in and binds to the UvrB monomer and, hence, forms a new UvrBC dimer. UvrB cleaves a phosphodiester bond 4 nucleotides downstream of the DNA damage, and the UvrC cleaves a phosphodiester bond 8 nucleotides upstream of the DNA damage and created 12 nucleotide excised segment. DNA helicase II (sometimes called UvrD) then comes in and removes the excised segment by actively breaking the hydrogen bonds between the complementary bases. The resultant gap is then filled in using DNA polymerase I and DNA ligase. The basic excision process is very similar in higher cells, but these cells usually involve many more proteins â€“ E.coli is a simple example.[5]
Telomere Telomeres are repetitive nucleotide sequences located at the termini of linear chromosomes of most eukaryotic organisms. For vertebrates, the sequence of nucleotides in telomeres is TTAGGG.[19] Most prokaryotes, having circular chromosomes rather than linear, do not have telomeres.[20] Telomeres compensate for incomplete semi-conservative DNA replication at chromosomal ends.[21] A protein complex known as shelterin serves to protect the ends of telomeres from being recognised as double-strand breaks by inhibiting homologous recombination (HR) and non-homologous end joining (NHEJ).[22][23]