New roles for cis-jasmone as an insect semiochemical and in plant defense

cis-jasmone, or (Z)-jasmone, is well known as a component of plant volatiles, and its release can be induced by damage, for example during insect herbivory. Using the olfactory system of the lettuce aphid to investigate volatiles from plants avoided by this insect, (Z)-jasmone was found to be electrophysiologically active and also to be repellent in laboratory choice tests. In field studies, repellency from traps was demonstrated for the damson-hop aphid, and with cereal aphids numbers were reduced in plots of winter wheat treated with (Z)-jasmone. In contrast, attractant activity was found in laboratory and wind tunnel tests for insects acting antagonistically to aphids, namely the seven-spot ladybird and an aphid parasitoid. When applied in the vapor phase to intact bean plants, (Z)-jasmone induced the production of volatile compounds, including the monoterpene (E)-beta-ocimene, which affect plant defense, for example by stimulating the activity of parasitic insects. These plants were more attractive to the aphid parasitoid in the wind tunnel when tested 48 h after exposure to (Z)-jasmone had ceased. This possible signaling role of (Z)-jasmone is qualitatively different from that of the biosynthetically related methyl jasmonate and gives a long-lasting effect after removal of the stimulus. Differential display was used to compare mRNA populations in bean leaves exposed to the vapor of (Z)-jasmone and methyl jasmonate. One differentially displayed fragment was cloned and shown by Northern blotting to be up-regulated in leaf tissue by (Z)-jasmone. This sequence was identified by homology as being derived from a gene encoding an alpha-tubulin isoform.

Liver transplantation after organ preservation with normothermic extracorporeal perfusion

OBJECTIVE

To study normothermic extracorporeal liver perfusion (NELP) as a means to preserve livers for transplantation and to reverse warm ischemic injury.

SUMMARY BACKGROUND DATA

The authors provide experimental evidence that successful transplantation after 4 hours of normothermic extracorporeal liver perfusion is possible and as reliable as 4 hours of cold preservation in University of Wisconsin solution. NELP preserves liver function completely and can reverse 60 minutes of warm ischemic injury in non-heart-beating donors.

METHODS

Thirty-six German Landrace pigs received transplants in six groups. Group 1 animals received direct transplantation. Group 2 received transplants after 4 hours of cold preservation with University of Wisconsin solution and Group 3 animals after 4 hours of NELP. Group 4 animals sustained 1 hour of warm ischemia before transplantation. Group 5 animals received transplants after 1 hour of warm ischemia and 4 hours of cold preservation and Group 6 animals after 1 hour of warm ischemia and 4 hours of NELP.

RESULTS

All animals receiving livers treated by NELP survived more than 7 days after the transplant (Groups 3 and 6). In contrast, all animals in Group 5 developed primary graft nonfunction within 24 hours after transplantation.

CONCLUSION

The technique of NELP holds the potential to keep a mammalian liver outside the body completely functional, possibly for more than 4 hours. NELP can be used for liver preservation before transplantation or for the use of organs from non-heart-beating donors.

Photoreceptor shedding is initiated by light in the frog retina

Frogs maintained on a diurnal light-dark cycle (14 hours light and 10 hours darkness) shed their rod photoreceptor outer segment tips shortly after the onset of light. Shedding is synchronous and occurs in about 25 percent of the rod photoreceptors each day. Prolonged exposure to total darkness decreases the amount of shedding, after which exposure to light results in a large burst of synchronous shedding. Thus in the frog retina, the synchronous shedding of rod outer segment tips is shown to be directly related to light stimulation.

H-2-restricted cytolytic T lymphocytes specific for HLA display T cell receptors of limited diversity

We previously showed that H-2Kd-restricted cytotoxic T lymphocyte (CTL) clones specific for a single nonapeptide derived from the Plasmodium berghei circumsporozoite (PbCS) protein displayed T cell receptors (TCRs) of highly diverse primary structure. We have now analyzed the TCR repertoire of CTLs that recognize a peptide derived from the human class I major histocompatibility complex (MHC) molecule HLA-Cw3 in association with the same murine class I MHC molecule H-2Kd. We first sequenced the TCR alpha and beta genes of the CTL clone Cw3/1.1 and, based on this genomic analysis, the TCR alpha and beta cDNA junctional regions of 23 independent H-2Kd-restricted CTL clones specific for HLA-Cw3. The results show that the TCR chains display very limited heterogeneity, both in terms of V alpha, J alpha, V beta, and J beta segments, and in terms of length and sequence of the CDR3 alpha and beta loops. The TCR repertoire used in vivo was then analyzed by harvesting CTL populations from the peritoneal cavity of immune mice. The peritoneal exudate lymphocytes (PELs) displayed HLA-Cw3-specific cytolytic activity in the absence of any stimulation in vitro. Remarkably, most of these freshly isolated PELs expressed TCRs that shared the same structural features as those from HLA-Cw3-reactive CTL clones. Thus, our results show that a peptide from HLA-Cw3 presented by H-2Kd selects CTLs that bear TCRs of very limited diversity in vivo. When taken together with the high diversity of the TCRs specific for the PbCS peptide, these findings suggest that natural tolerance to self peptides presented by class I MHC molecules may substantially reduce the size of the TCR repertoire of CTLs specific for antigenic peptides homologous to self.

Monoclonal antibodies raised against engineered soluble mouse T cell receptors and specific for V alpha 8-, V beta 2- or V beta 10-bearing T cells

We have characterized a panel of monoclonal antibodies (mAb) produced by immunizing rats with two distinct soluble mouse alpha/beta T cell receptor (TcR). Fifty mAb were found to react with the corresponding surface-bound TcR. Such observations suggest that the soluble TcR molecules used as immunogen are folded in a conformation similar to the native structure. Furthermore, the binding to T cells of four antibodies was found to correlate with the expression of the V alpha 8, V beta 2 or V beta 10 gene segments. Finally, staining of T lymphocytes from various mouse strains suggests that (a) the two anti-V alpha 8 antibodies recognize different epitopes, and each on only a fraction of V alpha 8+ cells; (b) the anti-V beta 10 mAb identifies a V beta 10 polymorphism among mouse strains, and (c) T cells expressing the V beta 2 or V beta 10 gene segments are not subject to major clonal deletion events induced by the major histocompatibility complex class II and Mls products which were tested.

Clonal analysis of human T cell activation by the Mycoplasma arthritidis mitogen (MAS)

Mycoplasma arthritidis produces an as yet undefined soluble molecule (MAS) that has a potent mitogenic effect on T cells of several species. We have used cloned human cytotoxic and proliferative T lymphocytes to dissect the molecular mechanism of T cell activation by this mitogen. Reactivity to MAS is clonally expressed among T cell receptor (TcR) alpha/beta chain-expressing T cell clones of CD4+ or CD8+ phenotype, as well as CD4-8- TcR alpha/beta chain-negative T lymphocyte clones expressing the CD3-associated TcR gamma chain. MAS is able to induce cytotoxicity and/or proliferation in these T cell clones. For triggering of these T cells, regardless of their phenotype of specificity, the presence of autologous, allogeneic or xenogeneic major histocompatibility complex (MHC) class II molecules on accessory cells or target cells is necessary. However, T cells do not immunologically recognize MAS on class II molecules, since a direct action of MAS on the T cells themselves can be demonstrated. Triggering of T cells by MAS can be blocked by monoclonal antibodies against CD2, CD3 and the TcR alpha/beta chain dimer. We discuss as a possible explanation that MAS is a functionally bivalent molecule cross-linking TcR and MHC class II molecules. Thus, the mechanism of T cell activation by MAS has striking similarities to the mechanisms by which Staphylococcal enterotoxins activate T cells. It is intriguing that a similar mitogenic principle has been developed by two evolutionary distinct pathogenic microorganisms.

A selenium-containing phospholipid-hydroperoxide glutathione peroxidase in Schistosoma mansoni

The 100000Xg supernatant parasite platyhelminth Schistosoma mansoni exhibits a glutathione peroxidase activity with the substrate phosphatidylcholine hydroperoxide. Purification yielded a protein of 20 kDa molecular mass both on gel filtration column chromatography and SDS/PAGE, thus suggesting that S. mansoni expresses a protein similar to the mammalian selenoenzynic phospholipid-hydroperoxide glutathione peroxidase. Kinetic analysis and substrate specificity corroborated this assumption, the second-order rate constants for the oxidation of the ground-state enzyme (k+1) being higher with phosphatidylcholine hydroperoxide than with other peroxide substrates, such as cumene liydroperoxide or H2O2, and quantitatively similar to those of mammalian phospholipid-hydroperoxide glutathione peroxidase. Partial sequencing of the protein and selenium measurement by neutron activation analysis established that the purified peroxidase corresponded to the product of the S. mansoni gene previously reported and supposed to encode a selenium-containing glutathione peroxidase [Roche, C., Williams, D. L., Khalife, J., LePresle, T., Capron, A. & Pierce, R. J. (1994) Cloning and characterization of gene encoding Schistosoma mansoni glutathione peroxidase, Gene 138, 149 - 152]. S. mansoni thus contains a scienoperoxidase sharing molecular mass, catalytic efficiency and substrate specificity with phospholipid-hydroperoxide glutathione peroxidase, dismantling the concept that those enzymes are unique to vertebrate organisms.

4-Hydroxybenzoate hydroxylase from Pseudomonas sp. CBS3. Purification, characterization, gene cloning, sequence analysis and assignment of structural features determining the coenzyme specificity

4-Hydroxybenzoate hydroxylase from Pseudomonas sp. CBS3 was purified by five consecutive steps to apparent homogeneity. The enrichment was 50-fold with a yield of about 20%. The enzyme is a homodimeric flavoprotein monooxygenase with each 44-kDa polypeptide chain containing one FAD molecule as a rather weakly bound prosthetic group. In contrast to other 4-hydroxybenzoate hydroxylases of known primary structure, the enzyme preferred NADH over NADPH as electron donor. The pH optimum for catalysis was pH 8.0 with a maximum turnover rate around 45 degrees C. Chloride ions were inhibitory, and competitive with respect to NADH. 4-Hydroxybenzoate hydroxylase from Pseudomonas sp. CBS3 has a narrow substrate specificity. In addition to the transformation of 4-hydroxybenzoate to 3,4-dihydroxybenzoate, the enzyme converted 2-fluoro-4-hydroxybenzoate, 2-chloro-4-hydroxybenzoate, and 2,4-dihydroxybenzoate. With all aromatic substrates, no uncoupling of hydroxylation was observed. The gene encoding 4-hydroxybenzoate hydroxylase from Pseudomonas sp. CBS3 was cloned in Escherichia coli. Nucleotide sequence analysis revealed an open reading frame of 1182 bp that corresponded to a protein of 394 amino acid residues. Upstream of the pobA gene, a sequence resembling an E. coli promoter was identified, which led to constitutive expression of the cloned gene in E. coli TG1. The deduced amino acid sequence of Pseudomonas sp. CBS3 4-hydroxybenzoate hydroxylase revealed 53% identity with that of the pobA enzyme from Pseudomonas fluorescens for which a three-dimensional structure is known. The active-site residues and the fingerprint sequences associated with FAD binding are strictly conserved. This and the conservation of secondary structures implies that the enzymes share a similar three-dimensional fold. Based on an isolated region of sequence divergence and site-directed mutagenesis data of 4-hydroxybenzoate hydroxylase from P. fluorescens, it is proposed that helix H2 is involved in determining the coenzyme specificity.

Low Molecular Weight Heparin and Prevention of Postoperative Thrombosis in Abdominal Surgery

In a prospective, double-blind, randomized multicenter trial the efficacy and safety of low molecular weight heparin and unfractionated heparin were compared for the prevention of postoperative deep vein thrombosis in patients undergoing abdominal surgery. Six hundred and seventy-three patients were randomly allocated to the two prophylaxis groups; 20 of these, however, did not undergo surgery and did not receive any prophylaxis. Of the remaining 653 patients 323 received one subcutaneous injection of 3,000 anti-Xa units of low molecular weight heparin and 330 received subcutaneously 5,000 U heparin three times a day. Treatment was initiated 2 h preoperatively and continued for 7 to 10 days. The occurrence of DVT was determined by the 125I-labelled fibrinogen uptake test and phlebography. Venous thrombosis was diagnosed in 24 of 323 patients (7.4%) treated with low molecular weight heparin and in 26 of 330 patients (7.9%) treated with low-dose heparin. DVT of proximal veins was detected in four patients of the low molecular weight heparin group and in three patients of the low-dose heparin group. During the observation period three pulmonary emboli - one fatal and two non-fatal - occurred in patients receiving prophylaxis with low-dose heparin. No pulmonary embolism was found in patients treated with low molecular weight heparin. Both prophylactic schemes were well tolerated. Intra-and postoperative blood loss, incidence of wound hematoma, frequency and volume of intra- and postoperative blood transfusion were similar in both groups with a slight advantage for the low molecular weight heparin group. The results of this trial show that the investigated low molecular weight heparin is at least as effective and safe as low-dose heparin in preventing deep vein thrombosis in patients undergoing elective abdominal surgery.

Genetic predisposition for high stress reactivity amplifies effects of early-life adversity

A dysregulation of the hypothalamus-pituitary-adrenocortical (HPA) axis and the experience of early-life adversity are both well-established risk factors for the development of affective disorders, such as major depression. However, little is known about the interaction of these two factors in shaping endophenotypes of the disease. Here, we studied the gene-environment interaction of a genetic predisposition for HPA axis dysregulation with early-life stress (ELS), assessing the short-, as well as the long-lasting consequences on emotional behavior, neuroendocrine functions and gene expression profiles. Three mouse lines, selectively bred for either high (HR), intermediate (IR), or low (LR) HPA axis reactivity, were exposed to one week of ELS using the limited nesting and bedding material paradigm. Measurements collected during or shortly after the ELS period showed that, regardless of genetic background, ELS exposure led to impaired weight gain and altered the animals' coping behavior under stressful conditions. However, only HR mice additionally showed significant changes in neuroendocrine stress responsiveness at a young age. Accordingly, adult HR mice also showed lasting consequences of ELS, including hyperactive stress-coping, HPA axis hyperreactivity, and gene expression changes in the Crh system, as well as downregulation of Fkbp5 in relevant brain regions. We suggest that the genetic predisposition for high stress reactivity interacts with ELS exposure by disturbing the suppression of corticosterone release during a critical period of brain development, thus exerting lasting programming effects on the HPA axis, presumably via epigenetic mechanisms. In concert, these changes lead to the emergence of important endophenotypes associated with affective disorders.

Molecular characterization of mouse T-cell ecto-ADP-ribosyltransferase Rt6: cloning of a second functional gene and identification of the Rt6 gene products

RT6 is an enzymatically active GPI-anchored membrane protein that was originally discovered in the rat as a peripheral T cell alloantigen. It has attracted interest as an activation antigen and because defective RT6-expression coincides with increased susceptibility for autoimmune type I diabetes in the BB rat. Southern blot analyses indicate that the rat carries a single copy RT6 gene whereas the mouse carries a duplication of the homologous locus. We had previously cloned and sequenced a RT6-homologous cDNA from BALB/c mouse spleen. We now report the cloning and characterization of a second RT6-homologue from BALB/c and 129/Sv mice. The two mouse Rt6 genes (designated Rt6-1 and Rt6-2) encode similar open reading frames that are disrupted by conserved introns. The nucleotide sequences of the Rt6-1 and Rt6-2 coding regions show 87% sequence identity, the deduced amino acid sequences 79% identity. The amino acid sequences reveal significant similarity to recently cloned ADP-ribosylating ectoenzymes from rabbit and human skeletal muscle as well as chicken bone marrow cells. RT-PCR analyses reveal that the two Rt6 genes are differentially expressed in distinct inbred mouse strains and that their transcripts are properly processed. Western blot analyses demonstrate that the respective gene products are released from cells by treatment with PI-PLC. The results further show that both mouse Rt6 genes are translated into GPI-anchored cell surface molecules and that Rt6 gene expression is restricted to peripheral lymphoid tissues.

Plant stress signalling: understanding and exploiting plant-plant interactions

When plants are attacked by insects, volatile chemical signals can be released, not only from the damaged parts, but also systemically from other parts of the plant and this continues after cessation of feeding by the insect. These signals are perceived by olfactory sensory mechanisms in both the herbivorous insects and their parasites. Molecular structures involved can be characterized by means of electrophysiological assays, using the insect sensory system linked to chemical analysis. Evidence is mounting that such signals can also affect neighbouring intact plants, which initiate defence by the induction of further signalling systems, such as those that increase parasitoid foraging. Furthermore, insect electrophysiology can be used in the identification of plant compounds having effects on the plants themselves. It has been found recently that certain plants can release stress signals even when undamaged, and that these can cause defence responses in intact plants. These discoveries provide the basis for new crop protection strategies, that are either delivered by genetic modification of plants or by conventionally produced plants to which the signal is externally applied. Delivery can also be made by means of mixed seed strategies in which the provoking and recipient plants are grown together. Related signalling discoveries within the rhizosphere seem set to extend these approaches into new ways of controlling weeds, by exploiting the elusive potential of allelopathy, but through signalling rather than by direct physiological effects.

[Removal of bone cement with laser]

In operations requiring replacement of cemented endoprothesis, the removal of both the prosthesis and the cement is often difficult as the cement adheres strongly to the bone. Mechanical removal frequently results in fenestration or traumatisation of the bone. The aim of non-contact removal of polymethylmethacrylate (PMMA) with the laser, is to access normally inaccessible regions while inflicting a minimum amount of damage to the bone substance. The much cited cw or superpulsed CO2-laser cannot be used clinically, due to the thermal stressing of the bone. The paper shows spectra of PMMA with and without dopants, e.g. Tinuvin as UV absorber, optical staining with a high-pressure mercury lamp at lambda = 275 +/- 25 nm, lambda = 350 +/- 25 nm and various radiation times, as well as with an excimer laser lambda = 248 nm, FWHM 20 ns, and ablation measurements were made with the following lasers: excimer laser, Lambda Physics, EMG 102, FWHM 25 ns, lambda = 351 nm, excimer laser, Technolas, MAX 10, FWHM 60 ns, lambda = 308 nm, and a pulsed CO2 laser from PSI, lambda = 9.2 and 10.6 microns, FWHM 130 and 65 microseconds, pulse peak power 3.8 and 7.7 kW. The excimer laser, pulse length less than 100 ns, is unsuitable for clinical use because the required removal rate cannot be achieved either with doped PMMA or with pure PMMA. More promising results have been obtained with the pulsed (microseconds range) CO2 laser which has a removal rate of up to 30 times that of the above-mentioned excimer laser, with significantly lower thermal stressing of the bone than with the cw or super pulsed CO2 laser.

Congruent representation of visual and acoustic space in the superior colliculus of the echolocating bat Phyllostomus discolor

The midbrain superior colliculus (SC) commonly features a retinotopic representation of visual space in its superficial layers, which is congruent with maps formed by multisensory neurons and motor neurons in its deep layers. Information flow between layers is suggested to enable the SC to mediate goal-directed orienting movements. While most mammals strongly rely on vision for orienting, some species such as echolocating bats have developed alternative strategies, which raises the question how sensory maps are organized in these animals. We probed the visual system of the echolocating bat Phyllostomus discolor and found that binocular high acuity vision is frontally oriented and thus aligned with the biosonar system, whereas monocular visual fields cover a large area of peripheral space. For the first time in echolocating bats, we could show that in contrast with other mammals, visual processing is restricted to the superficial layers of the SC. The topographic representation of visual space, however, followed the general mammalian pattern. In addition, we found a clear topographic representation of sound azimuth in the deeper collicular layers, which was congruent with the superficial visual space map and with a previously documented map of orienting movements. Especially for bats navigating at high speed in densely structured environments, it is vitally important to transfer and coordinate spatial information between sensors and motor systems. Here, we demonstrate first evidence for the existence of congruent maps of sensory space in the bat SC that might serve to generate a unified representation of the environment to guide motor actions.

Mechanism of replication of bacteriophage phi X174 XIX. Initiation of phi X174 viral strand DNA synthesis at internal sites on the genome

Bacteriophage phi X174 viral strand DNA molecules shorter than genome length found late in the infectious cycle in Escherichia coli were 5' end labeled with 32P. Hybridization of the 32P-labeled molecules to restriction enzyme fragments of phi X replicative form DNA revealed an excess of phi X molecules whose 5' ends mapped in HaeIII fragments Z3 and Z4 in comparison with fragments Z1 and Z2. This suggests that initiation of phi X174 viral strand DNA synthesis may occur at internal sites on the complementary strand. There are several appropriately located sequences that might serve as n' (factor Y) recognition sequences and thereby facilitate discontinuous synthesis of the viral strand.

99mTc-phosphonates for imaging of amyloid in C-cell carcinoma

A patient with recurrent C-cell carcinoma of the thyroid is presented. Tumor masses and metastatic lymph nodes were detected by 99mTc-DPD on a preoperative bone scan. In contrast to other causes of extraosseous accumulation of bone-seeking phosphonates, the high affinity of amyloid is the main factor in the case of C-cell carcinoma. Because amyloid is the typical histochemical sign of the carcinoma type, imaging with phosphonates is expected to be another specific diagnostic procedure in addition to calcitonin measurements. The role of other markers in thyroid carcinomas is discussed.

Xenogeneic, extracorporeal liver perfusion in primates improves the ratio of branched-chain amino acids to aromatic amino acids (Fischer's ratio)

In fulminant hepatic failure (FHF), the development of hepatic encephalopathy is associated with grossly abnormal concentrations of plasma amino acids (PAA). Normalization of the ratio of branched-chain amino acids to aromatic amino acids (Fischer's ratio) correlates with clinical improvement. This study evaluated changes in PAA metabolism during 4 h of isolated, normothermic extracorporeal liver perfusion using a newly designed system containing human blood and a rhesus monkey liver. Bile and urea production were within the physiological range. Release of the transaminases AST, ALT and LDH were minimal. The ratio of branched (valine, leucine, isoleucine) to aromatic (tyrosine, phenylalanine) amino acids increased significantly. These results indicate that a xenogeneic extracorporeal liver perfusion system is capable of significantly increasing Fischer's ratio and may play a role in treating and bridging patients in FHF in the future.

[Gastrointestinal bleeding in liver cirrhosis at the ICU]

Due to portal hypertension and bleeding disorders, patients with liver cirrhosis are at increased risk for severe gastrointestinal bleedings (GIB), commonly requiring therapy at the intensive care unit (ICU). In order to identify epidemiological and prognostic factors for GIB in cirrhotic patients, we retrospectively analysed patients from our medical ICU from 1999 to 2010. Among 7376 critically ill patients, 650 (8.8 %) were diagnosed with liver cirrhosis. Hepatic cirrhosis was frequently found in ICU patients admitted due to severe GIB (23.2 % of 711 patients had cirrhosis). Moreover, patients with cirrhosis were at increased risk to develop severe GIB during intensive care treatment (40.9 % of 44 patients with GIB during ICU stay had cirrhosis). Besides the high rate of variceal bleedings (64.4 %) in cirrhotic patients, non-variceal haemorrhages were also common (28.5 %). We identified the MELD score and necessity of mechanical ventilation as independent risk factors for mortality in cirrhotic patients with severe GIB. Patients with liver cirrhosis and severe GIB had significantly impaired prognosis (case-related fatality rate of 26.1 % with cirrhosis vs. 6.8 % without cirrhosis), especially in cases of newly developed GIB during ICU therapy. Advanced therapeutic approaches and novel strategies are warranted to improve the critical prognosis of these high-risk patients.

[Different effectiveness of two preparations of low molecular weight heparin in patients with elective hip joint replacement]

In an open randomised trial 3 regimen of prophylaxis for deep vein thrombosis were compared in 105 patients undergoing elective hip arthroplasty. One part of the patients received one injection daily of a low molecular weight heparin, another part was treated with one injection of a combination containing a different type of low molecular weight heparin + dihydroergotamine per day and a third part of the patients received 3 applications of 5000 IU unfractionated heparin per day. The frequency of DVT determined by the 125iodine-labelled fibrinogen uptake test and phlebography was 22.9% in patients receiving the combination and 50.0% in those receiving the low-molecular weight heparin alone. The difference was statistically significant with 2 p = 0.03. The incidence of DVT in patients receiving unfractionated heparin was 30.6%. No difference in the development of bleeding complications was found between the three treatment groups.

Mechanism of replication of bacteriophage phi X174 XX. Sensitivity of nascent DNA to single-strand-specific nucleases

We reported earlier that dephosphorylated nascent phi X174 viral strand DNA molecules were less extensively degraded from the 5' end by spleen exonuclease than were non-nascent molecules. Experiments described here revealed that the insensitivity to the 5'-OH end-specific nuclease was more evident among the longer molecules in the population than among the shorter, all of the molecules being less than unit length in size. The smallest molecules in the population were about as sensitive to the enzyme as the control molecules and hence must possess unblocked 5'-terminal nucleotides. Degradation of the nascent DNA with the 3' end-specific snake venom phosphodiesterase revealed only a small enrichment for [3H]thymidine near the 3' end, seemingly insufficient to account completely for the apparent insensitivity of the longer molecules to spleen exonuclease. When the nascent molecules were isolated without the use of proteolytic enzymes, some pronase-sensitive material was found associated with the DNA, particularly the longer molecules. We suggest that the resistance of the longer nascent (pronase-treated) molecules to spleen exonuclease occurs because they have remnants of the viral gene A or A* protein covalently bound to the 5' end.