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Abstract
We have isolated host mutants of Saccharomyces cerevisiae in which the 2µm plasmid is poorly maintained. All the mutants tested constituted one complementation group, which was designated map1 (maintenance of plasmid). Minichromosomes carrying a chromosomal replication origin and a centromere were affected in the mutants. Two types of hybrid plasmids generated in vivo and in vitro appeared to compensate for the mutations and had DNA regions containing multiple ARS (autonomously replicating sequence) or a set of 2µm inverted repeat sequences. These results suggested that poor maintenance of plasmids was due to low levels of replication, probably at the initiation of replication.