When pol I goes into high gear: processive DNA synthesis by pol I in the cell

Pol I is the most abundant polymerase in E. coli and plays an important role in short patch repair. In accord with this role in the cell, the purified polymerase exhibits low processivity and high fidelity in vitro. Pol I is also the polymerase responsible for leader strand synthesis during ColE1 plasmid replication. In a previous publication, we described the generation of a highly error-prone DNA polymerase I. Expression of this mutant Pol I results in errors during the replication of a ColE1 plasmid. The distribution and spectrum of mutations in the ColE1 plasmid sequence downstream the ori indicates that Pol I is capable of more processive replication in vivo than previously accepted. Here, we review evidence suggesting that Pol I may be recruited into a replisome-like holoenzyme and speculate that processive DNA replication by Pol I may play a role in recombination-dependent DNA replication in the cell.     

Template switches during plus-strand DNA synthesis of duck hepatitis B virus are influenced by the base composition of the minus-strand terminal redundancy

Two template switches are necessary during plus-strand DNA synthesis of the relaxed circular (RC) form of the hepadnavirus genome. The 3' end of the minus-strand DNA makes important contributions to both of these template switches. It acts as the donor site for the first template switch, called primer translocation, and subsequently acts as the acceptor site for the second template switch, termed circularization. Circularization involves transfer of the nascent 3' end of the plus strand from the 5' end of the minus-strand DNA to the 3' end, where further elongation can lead to production of RC DNA. In duck hepatitis B virus (DHBV), a small terminal redundancy (5'r and 3'r) on the ends of the minus-strand DNA has been shown to be important, but not sufficient, for circularization. We investigated what contribution, if any, the base composition of the terminal redundancy made to the circularization process. Using a genetic approach, we found a strong positive correlation between the fraction of A and T residues within the terminal redundancy and the efficiency of the circularization process in those variants. Additionally, we found that the level of in situ priming increases, at the expense of primer translocation, as the fraction of A and T residues in the 3'r decreases. Thus, a terminal redundancy rich in A and T residues is important for both plus-strand template switches in DHBV.     

Control of life, death, and differentiation in cultured midgut cells of the lepidopteran, Heliothis virescens

Summary Differentiated cells in the insect midgut depend on stem cells for renewal. We have immunologically identified Integrin β₁, a promotor of cell-cell adhesion that also induces signals mediating proliferation, differentiation, and apoptosis on the surfaces of culturedHeliothis virescens midgut cells; clusters of immunostained integrin β₁-like material, indicative of activated integrin, were detected on aggregating midgut columnar cells. Growth factor-like peptides (midgut differentiation factors 1 and 2 [MDF1 and MDF2]), isolated from conditioned medium containingManduca sexta midgut cells, may be representative of endogenous midgut signaling molecules. Exposing the cultured midgut cells toBacillus thuringiensis (Bt) toxin caused large numbers of mature differentiated cells to die, but the massive cell death simultaneously induced a 150–200% increase in the numbers of midgut stem and differentiating cells. However, after the toxin was washed out, the proportions of cell types returned to near-control levels within 2 d, indicating endogenous control of cell-population dynamics. MDF1 was detected immunologically in larger numbers of Bt-treated columnar cells than controls, confirming its role in inducing the differentiation of rapidly produced stem cells. However, other insect midgut factors regulating increased proliferation, differentiation, as well as inhibition of proliferation and adjustment of the ratio of cell types, remain to be discovered. Products mentioned in this article are not endorsed by the U.S. Department of Agriculture.     

Peptides that elicit midgut stem cell differentiation isolated from chymotryptic digests of hemolymph from Lymantria dispar pupae

Isolated stem cells of Heliothis virescens, cultured in vitro, were induced to differentiate by Midgut Differentiation Factors 3 and 4. These were peptides identified from a chymotrypsin digest of hemolymph taken from newly pupated Lymantria dispar. Partial purification was obtained by filtration through size exclusion filters. The most active preparation was subsequently subjected to a series of 3 Reverse Phase-HPLC procedures. Partial sequences of the peptides were identified via automated Edman degradation as the nanomers EEVVKNAIA-OH (MDF 3) and ITPTSSLAT-OH (MDF 4). These sequences were commercially synthesized. The synthetic compounds proved active in a dose-dependent manner. Stem cells responded to synthetic MDF 3 and MDF 4 as they did to previously identified peptides MDF 1 and 2, which have quite different amino acid sequences. All of the 4 MDFs administered singly induced statistically similar differentiation responses at 2 x 10(-8), 2 x 10(-9), and 2 x 10(-10) M. However, pairs of the 4 MDFs produced even more differentiation, the same response as one alone, no response, or were inhibitory, dependent on the MDF pair and its concentration. The data suggests complicated receptor interactions.     

Distribution of mutations in human thymidylate synthase yielding resistance to 5-fluorodeoxyuridine

Thymidylate synthase (TS) catalyzes methylation of dUMP to dTMP and is the target of cancer chemotherapeutic agents (e.g. 5-fluorouracil). Here, we used error-prone PCR to mutagenize the full-length human TS cDNA and then selected mutants resistant to 5-fluorodeoxyuridine in a bacterial complementation system. We found that resistant mutants contained 1-5 amino acid substitutions and that these substitutions were located along the entire length of the polypeptide. Mutations were frequent near the active site Cys(195) and in the catalytically important Arg(50) loop; however, many mutations were also distributed throughout the remainder of the cDNA. Mutants containing a single amino acid replacement identified the following 14 residues as unreported sites of resistance: Glu(23), Thr(51), Thr(53), Val(84), Lys(93), Asp(110), Asp(116), Pro(194), Ser(206), Met(219), His(250), Asp(254), Tyr(258), and Lys(284). Many of these residues are distant from the active site and/or have no documented function in catalysis or resistance. We conclude that mutations distributed throughout the linear sequence and three-dimensional structure of human TS can confer resistance to 5-fluorodeoxyuridine. Our findings imply that long range interactions within proteins affect catalysis at the active site and that mutations at a distance can yield variant proteins with desired properties.     

Formation of carnosine-Cu(II) complexes prevents and reverts the inhibitory action of copper in P2X4 and P2X7 receptors

To further analyze the action of copper on brain synaptic mechanisms, the brain dipeptide carnosine (beta-alanyl-L-histidine) was tested in Xenopus laevis oocytes expressing the rat P2X4 or P2X7 receptors. Ten micromolar copper halved the currents evoked by ATP in both receptors; co-application of carnosine plus copper prevented the metal induced-inhibition with a median effective concentration of 12.1 +/- 3.9 and 12.0 +/- 5.5 microm for P2X4 and P2X7, respectively. Zinc potentiated only the P2X4 ATP-evoked currents; carnosine had no effect over this metal. The relative potency and selectivity of classical metal chelators to prevent the copper inhibition was compared between carnosine and penicillamine (PA), bathophenanthroline (BPh) or L-histidine (His). Their rank order of potency in P2X4 and P2X7 receptors was carnosine = PA = His > BPh > Glycine (Gly) and carnosine = BPh = His > PA > Gly, respectively. The potency to prevent the zinc-induced potentiation in the P2X4 receptor was BPh > PA > His; carnosine, Gly and beta-alanine were inactive. Whereas 1-100 microm carnosine or His alone did not modify the ATP-evoked currents, 10-100 microm PA augmented and 100 microm BPh decreased the ATP-evoked currents. Carnosine was able to revert the copper-induced inhibition restoring the maximal ATP gated current in a concentration-dependent manner. Electronic spectroscopy confirm the formation of carnosine-Cu(II) complexes, mechanism that can account for the prevention and reversal of the copper inhibition, revealing its potential in copper intoxication treatment.     

Stem cells from midguts of Lepidopteran larvae: clues to the regulation of stem cell fate

Previously, we showed that isolated stem cells from midguts of Heliothis virescens can be induced to multiply in response to a multiplication protein (MP) isolated from pupal fat body, or to differentiate to larval types of mature midgut cells in response to either of 4 differentiation factors (MDFs) isolated from larval midgut cell-conditioned medium or pupal hemolymph. In this work, we show that the responses to MDF-2 and MP in H. virescens stem cells decayed at different time intervals, implying that the receptors or response cascades for stem cell differentiation and multiplication may be different. However, the processes appeared to be linked, since conditioned medium and MDF-2 prevented the action of MP on stem cells; MP by itself appeared to repress stem cell differentiation. Epidermal growth factor, retinoic acid, and platelet-derived growth factor induced isolated midgut stem cells of H. virescens and Lymantria dispar to multiply and to differentiate to mature midgut cells characteristic of prepupal, pupal, and adult lepidopteran midgut epithelium, and to squamous-like cells and scales not characteristic of midgut tissue instead of the larval types of mature midgut epithelium induced by the MDFs. Midgut stem cells appear to be multipotent and their various differentiated fates can be influenced by several growth factors.