RESUMO
Analysis of a 131-kb segment of the left arm of yeast chromosome XIV beginning 157 kb from the telomere reveals four highly active origins of replication that initiate replication late in S phase. Previous work has shown that telomeres act as determinants for late origin activation. However, at least two of the chromosome XIV origins maintain their late activation time when located on large circular plasmids, indicating that late replication is independent of telomeres. Analysis of the replication time of plasmid derivatives containing varying amounts of chromosome XIV DNA show that a minimum of three chromosomal elements, distinct from each tested origin, contribute to late activation time. These late determinants are functionally equivalent, because duplication of one set of contributing sequences can compensate for the removal of another set. Furthermore, insertion of an origin that is normally early activated into this domain results in a shift to late activation, suggesting that the chromosome XIV origins are not unique in their ability to respond to the late determinants.
Assuntos
Cromossomos Fúngicos/genética , Replicação do DNA/genética , Origem de Replicação/genética , Fase S/genética , Saccharomyces cerevisiae/genética , Sequência de Bases , Passeio de Cromossomo , Clonagem Molecular , DNA Fúngico/biossíntese , DNA Fúngico/genética , Modelos Genéticos , Dados de Sequência Molecular , Plasmídeos/biossíntese , Plasmídeos/genética , Mapeamento por Restrição , Análise de Sequência de DNARESUMO
Replication origins facilitate the choreography of genome duplication by acting as targets for regulatory mechanisms. Eukaryotic cells control the efficiency and the time of origin activity. In addition, origins that have been replicated are prevented from doing so twice in the same cell cycle. In this review we will examine the mechanisms that may be used to control these processes, and discuss the role of replication in regulating transcription.
Assuntos
Células Eucarióticas , Origem de Replicação/fisiologia , Animais , Replicação do DNA/genética , Replicação do DNA/fisiologia , Origem de Replicação/genética , Transcrição GênicaRESUMO
The nature of replication origins in eukaryotic chromosomes has been examined in some detail only in yeast, Drosophila, and mammalian cells. We have used highly synchronous cultures of plasmodia of the myxomycete Physarum and two-dimensional agarose gel electrophoresis to examine replication of two developmentally controlled, early replicated genes over time in S-phase. A single, discrete origin of replication was found within 4.8 kb of the LAV1-5 gene, which encodes a homolog of profilin. In contrast, the LAV1-2 gene appears to be surrounded by several origins. Two origins were identified within a 15 kb chromosomal domain and appear to be inefficiently used. Replication forks collide at preferred sites within this domain. These terminating structures are long lived, persisting for at least 2 h of the 3 h S-phase. Analysis of restriction fragment length polymorphisms (RFLPs) within the LAV1-2 domain indicates that replication of alleles on different parental chromosomes is a highly coordinated process. Our studies of the these two early replicated, plasmodium-specific genes indicate that both a fixed, narrow origin region and a broader zone containing two closely spaced origins of DNA replication occur in Physarum.
