Characterization of an alpha-like DNA polymerase from Bombyx mori silkglands

Characterisation of the DNA-polymerase-alpha-primase complex from the silk glands of Bombyx mori

Silk gland cells of Bombyx mori undergo chromosomal endoduplication throughout larval development. The DNA content of both posterior and middle silk gland nuclei increased by 300,000 times the haploid genomic content, amounting to 18 rounds of replication. The DNA doubling time is approximately 48 h and 24 h during the fourth and fifth instars of larval development. However, DNA content does not change during the interim moult. Concomitant with DNA content, DNA polymerase activity also increases as development progressed. Enzyme activity is predominantly due to DNA polymerase alpha with no detectable level of polymerase beta. DNA polymerase alpha from silk gland extracts was purified to homogeneity (using a series of columns involving ion-exchange, gel-filtration and affinity chromatography), resulting in a 4000-fold increase in specific activity. The enzyme is a heterogeneous multimer of high molecular mass, and the catalytic (polymerase) activity is resident in the 180-kDa subunit. The enzyme shows a pI of 6.2 and the Km values for the dNTP vary over 5-16 microM. The polymerase is tightly associated with primase activity and initiates primer synthesis in the presence of ribonucleoside triphosphates on a single-stranded DNA template. The primase activity is resident in the 45-kDa subunit. The enzyme is devoid of any detectable exonuclease activity. The abundance of DNA polymerase alpha in silk glands and its strong association with the nuclear matrix suggest a role in the DNA endoduplication process.

DNA polymerases alpha and gamma during pre-emergence and early larval development of Artemia

DNA polymerases alpha and gamma have been studied in cryptobiotic cysts and developing embryos and larvae of the brine shrimp Artemia. The two enzymes readily separate on Cibacron blue 3-GA Matrex gel. Assay requirements with activated DNA as primer-template are pH 8.0, 1 mM Mg2+, 50 mM K+ for DNA polymerase alpha and pH 8.4, 10 mM Mg2+, 80 mM K+ for DNA polymerase gamma. DNA polymerase alpha is inhibited by N-ethylmaleimide (94% and 100% at 1 mM and 10 mM respectively) and aphidicolin (96% at 60 microM). DNA polymerase gamma is also sensitive to N-ethylmaleimide (83% and 100% inhibition at 1 mM and 10 mM respectively) but is resistant to aphidicolin. 2',3'-Dideoxythymidine 5'-triphosphate (ddTTP) inhibits the gamma polymerase by 88% when in fivefold excess over dTTP whereas the alpha polymerase is unaffected by this compound. DNA polymerase alpha has a sedimentation coefficient of 7.6 S which is reduced to 6.2 S by a phenylmethylsulphonyl fluoride-sensitive proteinase. The gamma polymerase sediments at 8.3 S. No DNA polymerase beta activity could be detected. After the reinitiation of development both activities increased twofold up to 8 h (gamma polymerase) and 16 h (alpha polymerase), then declined before the onset of nuclear DNA replication after hatching. Thymidine kinase activity increased over 200-fold up to the time of replication. Analysis on Percoll density gradients of the intracellular distribution of both polymerases during development suggests that the changes in their activities may be due to migration from storage sites to replication complexes in the nuclei and mitochondria. The content of the mitochondrial DNA polymerase gamma in different batches of cysts may reflect the relative viabilities of the cysts.