DNA polymerase alpha and beta in the California urchin

Size difference in catalytic polypeptides of two active forms of mouse DNA polymerase alpha and separation of the primase subunit from one form, DNA replicase

There are two active forms of DNA polymerase alpha in mouse cells. One form (DNA replicase) is a DNA polymerase associated with primase activity and the other form (7.3 S polymerase) has no primase activity (Yaugar, T., Kozu, T. and Seno, T. (1982) J. Biol. Chem. 257, 11121-11127). The primase activity was dissociated from partially purified DNA replicase by hydroxyapatite column chromatography in buffer containing dimethyl sulfoxide and ethylene glycol. Nearly homogeneous primase, consisting of a 58 kDa polypeptide was obtained by glycerol gradient sedimentation and DEAE-cellulose column chromatography. Experiments on the effect of proteinase treatment and measurement of the molecular weight of the catalytic polypeptide of DNA replicase after its dissociation from the primase polypeptide indicated that the primase is not part of the DNA polymerase molecule, but an independent protein associated with DNA polymerase alpha, and that the latter is a 115 kDa catalytic polypeptide. The other form of DNA polymerase alpha, 7.3 S polymerase, consists of a 72 kDa catalytic polypeptide. Thus, the two forms of mouse DNA polymerase alpha have partially, if not completely, different catalytic polypeptide structures, suggesting that the 7.3 S polymerase is not simply formed from DNA replicase by dissociation of the primase subunit.

The small subunit of ribonucleotide reductase is encoded by one of the most abundant translationally regulated maternal RNAs in clam and sea urchin eggs

In both clam oocytes and sea urchin eggs, fertilization triggers the synthesis of a set of proteins specified by stored maternal mRNAs. One of the most abundant of these (p41) has a molecular weight of 41,000. This paper describes the identification of p41 as the small subunit of ribonucleotide reductase, the enzyme that provides the precursors necessary for DNA synthesis. This identification is based mainly on the amino acid sequence deduced from cDNA clones corresponding to p41, which shows homology with a gene in Herpes Simplex virus that is thought to encode the small subunit of viral ribonucleotide reductase. Comparison with the B2 (small) subunit of Escherichia coli ribonucleotide reductase also shows striking homology in certain conserved regions of the molecule. However, our attention was originally drawn to protein p41 because it was specifically retained by an affinity column bearing the monoclonal antibody YL 1/2, which reacts with alpha-tubulin (Kilmartin, J. V., B. Wright, and C. Milstein, 1982, J. Cell Biol., 93:576-582). The finding that this antibody inhibits the activity of sea urchin embryo ribonucleotide reductase confirmed the identity of p41 as the small subunit. The unexpected binding of the small subunit of ribonucleotide reductase can be accounted for by its carboxy-terminal sequence, which matches the specificity requirements of YL 1/2 as determined by Wehland et al. (Wehland, J., H. C. Schroeder, and K. Weber, 1984, EMBO [Eur. Mol. Biol. Organ.] J., 3:1295-1300). Unlike the small subunit, there is no sign of synthesis of a corresponding large subunit of ribonucleotide reductase after fertilization. Since most enzymes of this type require two subunits for activity, we suspect that the unfertilized oocytes contain a stockpile of large subunits ready for combination with newly made small subunits. Thus, synthesis of the small subunit of ribonucleotide reductase represents a very clear example of the developmental regulation of enzyme activity by control of gene expression at the level of translation.

Multiple polymorphic alpha- and beta-tubulin mRNAs are present in sea urchin eggs

Multiple alpha- and beta-tubulin RNAs were found in the mature unfertilized eggs of the sea urchin Lytechinus pictus. The alpha-tubulin RNAs were polymorphic in number, size, and relative amounts in the eggs of different females. Five to seven different size classes [1.75-4.2 kilobases (kb)] were detected on RNA gel blots. All egg preparations contained variable amounts of 1.8- and 2.25-kb beta-tubulin RNAs, and a few of them contained an additional 2.9-kb beta-tubulin RNA. The total amount of alpha-tubulin RNA did not always parallel that of beta-tubulin RNA. A portion of all of the various alpha- and beta-tubulin RNAs were polyadenylylated. RNase H digestions ruled out the possibility that some of these RNAs represented a single transcript bearing different lengths of 3' poly(A). One class of alpha-tubulin RNAs (2.4-2.65 kb) was reduced to 2 kb by RNase H, suggesting the presence of internal oligo(A) regions. All of the egg beta-tubulin RNAs sedimented as free ribonucleoprotein particles. Only a small portion of the 1.75- to 3.6-kb alpha-tubulin RNAs, but most of the 4.2-kb alpha-tubulin RNA, were found on polysomes before fertilization. In the 30-min embryo, small amounts of each of the various alpha- and beta-tubulin RNAs were recruited onto polysomes. Thus, each of the multiple polymorphic alpha- and beta-tubulin RNAs in the egg represent translationally competent mRNA.

Association of DNA polymerase alpha and beta with rough endoplasmic reticulum in sea-urchin eggs and changes in subcellular distribution during early embryogenesis

The subcellular distribution of DNA polymerase alpha and beta was examined in unfertilized eggs and embryos of the sea-urchin, Hemicentrotus pulcherrimus. In unfertilized egg homogenates, prepared in sucrose solution containing 5 mM 2-mercaptoethanol and 5 mM MgCl2, DNA polymerases equilibrated in isospycnic centrifugation as a single peak at a buoyant density of 1.261 g/cm3 (band I). This indicates that DNA polymerases associate with a cytoplasmic organelle. Band I was converted to bands II (1.227 g/cm3), III (1.177 g/cm3) and IV (1.146 g/cm3) by EDTA treatment. The RNA content of bands I, II, III and IV was approximately 0.34, 0.25, 0.10 and 0.04 mg/mg protein respectively. In isokinetic and isopycnic centrifugations both DNA polymerase and RNA cosedimented and coequilibrated. These results suggest that bands I, II, III and IV contain various amounts of ribosomes on a common structure. Examination by electron microscopy indicated that bands I, II, III and IV contained mainly monolayered membrane vesicles with different amounts of bound ribosomes. The content of ribosomes varied in the order: band I > band II > band III > band IV. Each band contained DNA polymerase with sedimentation coefficients of 5.8-7.6 S (sensitive to N-ethylmaleimide and aphidicolin) and 3.2 S (insensitive to these drugs). We conclude that almost all of DNA polymerases alpha and beta are localized on the rough endoplasmic reticulum in unfertilized eggs. Mixing experiments suggest that the association is specific and is not an artifact of homogenization. During development of embryos DNA polymerases associated with the membrane decreased with concomitant increase in the nuclear fraction. This suggests that the enzymes migrate from the cytoplasm to the nucleus. It is discussed that the role of the endoplasmic reticulum as a storage site of DNA polymerases in unfertilized eggs and the mechanism of translocation of DNA polymerases from the cytoplasm to the nucleus during early embryogenesis.

The mode of inhibitory action by pyridoxal 5-phosphate on DNA polymerase-alpha and -beta

The kinetics of the inhibition of DNA polymerases-alpha and -beta from sea urchin embryos by pyridoxal 5-phosphate were studied. The inhibition of DNA polymerase-alpha activity by pyridoxal 5-phosphate was competitive with activated DNA but noncompetitive with each deoxynucleoside triphosphate. With poly(dC)-oligo(dG)12-18 as a template-primer, however, the inhibition of DNA polymerase-alpha was competitive with dGTP but noncompetitive with the template-primer. These results suggest that DNA polymerase-alpha interacts with activated DNA and poly(dC)-oligo(dG)12-18 in different ways. The inhibition of DNA polymerase-beta by pyridoxal 5-phosphate was competitive with deoxynucleoside triphosphate using activated DNA as a template-primer and noncompetitive with activated DNA. Using poly(rA)-oligo(dT)12-18 as a template-primer, DNA polymerase-beta activity yielded sigmoid curves against both dTTP and the template-primer concentrations and was inhibited by pyridoxal 5-phosphate noncompetitively with respect to both dTTP and the template-primer. These results indicate that the inhibitory mode of DNA polymerase-alpha by pyridoxal 5-phosphate is different from that of DNA polymerase-beta.