ABSTRACT
DNA polymerase-α from embryonic chicken brain was resolved on DEAEcellulose into 3 component activities that remained distinct upon rechromatography. Product formation by each activity required exogenously added template-primer DNA, all 4 deoxynucleoside triphosphates, and a divalent metal cation. Each form incorporated [3H]- dTTP or [3H]-dCTP into a high molecular weight product that was identified as DNA by its chromatographic behavior and its sensitivity to DNase. High ionic strength, Nethylmaleimide, and the polymerase-α-specific inhibitor aphidicolin inhibited each activity; the apparent Ki, value of aphidicolin was 3·0 μΜ in each case. Based on these results, the 3 activities were identified as multiple forms of DNA polymerase-α . Experiments using embryonic chicken brains of various ages indicated that polymerase-α1, and polymerase-α3 reached maximal activity in 9-day-old embryos, while polymerase-α2 activity was elevated at a slightly later developmental stage. Using poly (dC) as template, high primase activity was detected in polymerase-α1, fractions.
ABSTRACT
Multiple forms of ricin have been isolated from castor bean seeds. Two forms, ricin-1 and ricin-2, differ in their isoelectric pI values and toxicity towards IMR- 32 cells. Inhibition of IMR-32 DNA polymerase α2 is more pronounced with ricin-1 (65%) than with ricin-2 (10%). Ricin Β chain (pI = 5.2) isolated from ricin-1 binds to IMR-32 cell surfaces as well as inhibits DNA polymerase α2 activity when studied in vitro. The presence of galβ-linked glycoconjugates near the active site of IMR-32 DNA polymerase α2 has been proposed. Replication modulators which bind to the glycose portion of the enzymes involved in the replication system may need a mandatory binding to cell surface glycoconjugates for their activity.