An autonomously replicating sequence (ARS) contains the origin of replication in the yeast genome. It contains four regions (A, B1, B2, and B3), named in order of their effect on plasmid stability; when these regions are mutated, replication does not initiate.
Element A is highly conserved, consisting of the consensus sequence:5'- T/A T T T A Y R T T T T/A -3'
(where Y is either pyrimidine and R is either purine). When this element is mutated, the ARS loses all activity.
As seen above the ARS are considerably A-T rich which makes it easy for replicative proteins to disrupt the H-bonding in that area. ORC protein complex (Origin Recognition Complex) is bound at the ARS throughout the cell cycle, allowing replicative proteins access to the ARS.
Mutational analysis for the yeast ARS elements have shown that any mutation in the B1, B2 and B3 regions result in a reduction of function of the ARS element. A mutation in the A region results in a complete loss of function.
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