Investigation of the bicinchoninic acid protein assay: identification of the groups responsible for color formation

The colored complex formed between Cu+ and bicinchoninic acid is the basis of the bicinchoninic acid protein assay (P. K. Smith, R. I. Krohn, G. T. Hermanson, A. K. Mallia, F. H. Gartner, M. D. Provenzano, E. K. Fujimoto, N. M. Goeke, B.J. Olson, and D.C. Klenk (1985) Anal. Biochem. 150, 76-85). Studies show that cysteine, tryptophan, tyrosine, and the peptide bond are capable of reducing Cu2+ to Cu+. Electrochemical studies and the magnitude of the color changes observed when the reaction is carried out at 37 degrees C indicate that tryptophan, tyrosine, and the peptide bond are not completely oxidized at this temperature. When the reaction temperature is increased to 60 degrees C, significantly more color formation is observed for these three groups. Studies with di-, tri-, and tetrapeptides and with proteins indicate that the extent of color formation is not the sum of the contributions of the individual color producing functional groups. Compounds with functional groups similar to those of cysteine, cystine, tyrosine, or tryptophan are shown to react with the bicinchoninic acid reagent. The color formed by these compounds in the presence of bovine serum albumin cannot be compensated for by using a reagent blank containing an identical concentration of the interfering compound.

Initial stages of tumor cell-induced angiogenesis: evaluation via skin window chambers in rodent models

BACKGROUND

There is a paucity of information about events that follow immediately after tumor cells are triggered to initiate the process of angiogenesis (the formation of new blood vessels). Such information is relevant to the issue of when micrometastases vascularize and has implications for the accessibility of micrometastases to various treatments. In this study, we attempted to monitor events at the initiation of angiogenesis at the earliest possible stage of tumor growth in vivo.

METHODS

Two different rodent mammary tumor cell lines, R3230Ac from the Fischer 344 rat and 4T1 from the BALB/c mouse, were stably transfected with a gene that encodes an enhanced version of green fluorescence protein (GFP). GFP-labeled R3230Ac or 4T1 cells (about 20-50 cells) were implanted into dorsal skinfold window chambers of Fischer 344 rats or BALB/c mice, respectively. Tumor angiogenesis was then monitored serially and noninvasively for up to 4 weeks.

RESULTS

Clear evidence of modification of the host vasculature was observed when tumor mass reached approximately 60-80 cells, and functional new blood vessels were seen when tumor mass reached roughly 100-300 cells. Individual tumor cells exhibited a chemotaxis-like growth pattern toward the pre-existing host vasculature. When ex-flk1 (a soluble, truncated vascular endothelial cell growth factor receptor protein known to be antiangiogenic) was injected with the tumor cells, the initial angiogenic and tumor growth activities were inhibited considerably, indicating that angiogenesis inhibitors may halt tumor growth even before the onset of angiogenesis.

CONCLUSION

Angiogenesis induced by tumor cells after implantation in the host begins at a very early stage, i.e., when the tumor mass contains roughly 100-300 cells. Identification of chemotactic signals that initiate tumor cell migration toward the existing vasculature may provide valuable targets for preventing tumor progression and/or metastases.

Comparison of tumor and normal tissue oxygen tension measurements using OxyLite or microelectrodes in rodents

In this study we compare oxygen tension (Po 2) histograms measured with O2 microelectrodes and a new optical Po 2 measurement device, the OxyLite, in normal tissues (mouse spleen and thymus) and in tumors (R3230Ac in rats) ( n = 5–6). The transient response to glucose infusion or 100% O2 breathing (hyperoxia) was also measured in tumors. Po 2 histograms of spleen and thymus with the two devices were not different. The OxyLite tumor Po 2 histogram, however, was left-shifted compared with the microelectrode (median Po 21.0 vs. 4.0 mmHg, P = 0.016). Both probes responded to acute hyperglycemia with a mean increase of 3–6 mmHg, but the microelectrode change was not significant. The OxyLite consistently recorded large Po 2 increases (∼28 mmHg) with hyperoxia, whereas the microelectrode response was variable. The OxyLite averages Po 2 over an area that contains interstitial and vascular components, whereas the microelectrode measures a more local Po 2. This study demonstrates the importance of considering the features of the measurement device when studying tissues with heterogeneous Po 2 distributions (e.g., tumors).

Acetic acid increases stability of silage under aerobic conditions

The effects of various compounds on the aerobic stability of silages were evaluated. It has been observed that inoculation of whole-crop maize with homofermentative lactic acid bacteria leads to silages which have low stability against aerobic deterioration, while inoculation with heterofermentative lactic acid bacteria, such as Lactobacillus brevis or Lactobacillus buchneri, increases stability. Acetic acid has been proven to be the sole substance responsible for the increased aerobic stability, and this acid acts as an inhibitor of spoilage organisms. Therefore, stability increases exponentially with acetic acid concentration. Only butyric acid has a similar effect. Other compounds, like lactic acid, 1,2-propanediol, and 1-propanol, have been shown to have no effect, while fructose and mannitol reduce stability.

Ascaris haemoglobin is a nitric oxide-activated 'deoxygenase'

The parasitic nematode Ascaris lumbricoides infects one billion people worldwide. Its perienteric fluid contains an octameric haemoglobin that binds oxygen nearly 25,000 times more tightly than does human haemoglobin. Despite numerous investigations, the biological function of this molecule has remained elusive. The distal haem pocket contains a metal, oxygen and thiol, all of which are known to be reactive with nitric oxide. Here we show that Ascaris haemoglobin enzymatically consumes oxygen in a reaction driven by nitric oxide, thus keeping the perienteric fluid hypoxic. The mechanism of this reaction involves unprecedented chemistry of a haem group, a thiol and nitric oxide. We propose that Ascaris haemoglobin functions as a 'deoxygenase', using nitric oxide to detoxify oxygen. The structural and functional adaptations of Ascaris haemoglobin suggest that the molecular evolution of haemoglobin can be rationalized by its nitric oxide related functions.

Oxygen distribution and consumption in the cat retina during normoxia and hypoxemia

Oxygen tension (PO2) was measured with microelectrodes within the retina of anesthetized cats during normoxia and hypoxemia (i.e., systemic hypoxia), and photoreceptor oxygen consumption was determined by fitting PO2 measurements to a model of steady-state oxygen diffusion and consumption. Choroidal PO2 fell linearly during hypoxemia, about 0.64 mmHg/mmHg decrease in arterial PO2 (PaO2). The choroidal circulation provided approximately 91% of the photoreceptors' oxygen supply under dark-adapted conditions during both normoxia and hypoxemia. In light adaptation the choroid supplied all of the oxygen during normoxia, but at PaO2's less than 60 mmHg the retinal circulation supplied approximately 10% of the oxygen. In the dark-adapted retina the decrease in choroidal PO2 caused a large decrease in photoreceptor oxygen consumption, from approximately 5.1 ml O2/100 g.min during normoxia to 2.6 ml O2/100 g.min at a PaO2 of 50 mmHg. When the retina was adapted to a rod saturating background, normoxic oxygen consumption was approximately 33% of the dark-adapted value, and hypoxemia caused almost no change in oxygen consumption. This difference in metabolic effects of hypoxemia in light and dark explains why the standing potential of the eye and retinal extracellular potassium concentration were previously found to be more affected by hypoxemia in darkness. Frequency histograms of intraretinal PO2 were used to characterize the oxygenation of the vascularized inner half of the retina, where the oxygen distribution is heterogeneous and simple diffusion models cannot be used. Inner retinal PO2 during normoxia was relatively low: 18 +/- 12 mmHg (mean and SD; n = 8,328 values from 36 profiles) in dark adaptation, and significantly lower, 13 +/- 6 mmHg (n = 4,349 values from 19 profiles) in light adaptation. Even in the dark-adapted retina, 30% of the values were less than 10 mmHg. The mean PO2 in the inner (i.e., proximal) half of the retina was well regulated during hypoxemia. In dark adaptation it was significantly reduced only at PaO2's less than 45 mmHg, and it was reduced less at these PaO2's in light adaptation.

Impaired vascular contractility and blood pressure homeostasis in the smooth muscle alpha-actin null mouse

The smooth muscle (SM) alpha-actin gene activated during the early stages of embryonic cardiovascular development is switched off in late stage heart tissue and replaced by cardiac and skeletal alpha-actins. SM alpha-actin also appears during vascular development, but becomes the single most abundant protein in adult vascular smooth muscle cells. Tissue-specific expression of SM alpha-actin is thought to be required for the principal force-generating capacity of the vascular smooth muscle cell. We wanted to determine whether SM alpha-actin gene expression actually relates to an actin isoform's function. Analysis of SM alpha-actin null mice indicated that SM alpha-actin is not required for the formation of the cardiovascular system. Also, SM alpha-actin null mice appeared to have no difficulty feeding or reproducing. Survival in the absence of SM alpha-actin may result from other actin isoforms partially substituting for this isoform. In fact, skeletal alpha-actin gene, an actin isoform not usually expressed in vascular smooth muscle, was activated in the aortas of these SM alpha-actin null mice. However, even with a modest increase in skeletal alpha-actin activity, highly compromised vascular contractility, tone, and blood flow were detected in SM alpha-actin-defective mice. This study supports the concept that SM alpha-actin has a central role in regulating vascular contractility and blood pressure homeostasis, but is not required for the formation of the cardiovascular system.

Antibodies against the human papillomavirus type 16 early proteins in human sera: correlation of anti-E7 reactivity with cervical cancer

By Western blot technique, 519 samples of human sera were tested for the presence of antibodies to the human papillomavirus (HPV) type 16 proteins E4 and E7 that had been expressed in Escherichia coli as fusion proteins. Sera were obtained from patients attending the University hospitals for reasons unrelated to HPV infections (controls), from patients with HPV-associated lesions, as well as from patients suffering from cervical cancer. Within the control population, 18.1% of them had antibodies that reacted with the E4 protein, and 3.9% of them had antibodies that reacted with the E7 protein. No sex-specific difference in the antibody prevalence was observed. The highest proportion of anti-E4 antibody-positive individuals (40.7%) was observed in the age group between 11 and 20 years. The frequency of anti-E4-positive sera was threefold higher in patients with HPV-associated genital lesions than that in age-matched controls. Antibodies against the HPV16 E7 protein were found 14 times more frequently in patients with cervical cancer, compared with age- and sex-matched controls (P less than .00001). From these data, we concluded that anti-E4 antibodies may be correlated with virus replication and that anti-E7 antibodies may represent a marker for cervical cancer development.

Quantification of longitudinal tissue pO2 gradients in window chamber tumours: impact on tumour hypoxia

We previously reported that the arteriolar input in window chamber tumours is limited in number and is constrained to enter the tumour from one surface, and that the pO2 of tumour arterioles is lower than in comparable arterioles of normal tissues. On average, the vascular pO2 in vessels of the upper surface of these tumours is lower than the pO2 of vessels on the fascial side, suggesting that there may be steep vascular longitudinal gradients (defined as the decline in vascular pO2 along the afferent path of blood flow) that contribute to vascular hypoxia on the upper surface of the tumours. However, we have not previously measured tissue pO2 on both surfaces of these chambers in the same tumour. In this report, we investigated the hypothesis that the anatomical constraint of arteriolar supply from one side of the tumour results in longitudinal gradients in pO2 sufficient in magnitude to create vascular hypoxia in tumours grown in dorsal flap window chambers. Fischer-344 rats had dorsal flap window chambers implanted in the skin fold with simultaneous transplantation of the R3230AC tumour. Tumours were studied at 9-11 days after transplantation, at a diameter of 3-4 mm; the tissue thickness was 200 microm. For magnetic resonance microscopic imaging, gadolinium DTPA bovine serum albumin (BSA-DTPA-Gd) complex was injected i.v., followed by fixation in 10% formalin and removal from the animal. The sample was imaged at 9.4 T, yielding voxel sizes of 40 microm. Intravital microscopy was used to visualize the position and number of arterioles entering window chamber tumour preparations. Phosphorescence life time imaging (PLI) was used to measure vascular pO2. Blue and green light excitations of the upper and lower surfaces of window chambers were made (penetration depth of light approximately 50 vs >200 microm respectively). Arteriolar input into window chamber tumours was limited to 1 or 2 vessels, and appeared to be constrained to the fascial surface upon which the tumour grows. PLI of the tumour surface indicated greater hypoxia with blue compared with green light excitation (P < 0.03 for 10th and 25th percentiles and for per cent pixels < 10 mmHg). In contrast, illumination of the fascial surface with blue light indicated less hypoxia compared with illumination of the tumour surface (P < 0.05 for 10th and 25th percentiles and for per cent pixels < 10 mmHg). There was no significant difference in pO2 distributions for blue and green light excitation from the fascial surface nor for green light excitation when viewed from either surface. The PLI data demonstrates that the upper surface of the tumour is more hypoxic because blue light excitation yields lower pO2 values than green light excitation. This is further verified in the subset of chambers in which blue light excitation of the fascial surface showed higher pO2 distributions compared with the tumour surface. These results suggest that there are steep longitudinal gradients in vascular pO2 in this tumour model that are created by the limited number and orientation of the arterioles. This contributes to tumour hypoxia. Arteriolar supply is often limited in other tumours as well, suggesting that this may represent another cause for tumour hypoxia. This report is the first direct demonstration that longitudinal oxygen gradients actually lead to hypoxia in tumours.

Direct Demonstration of Instabilities in Oxygen Concentrations within the Extravascular Compartment of an Experimental Tumor

To test the hypothesis that temporal variations in microvessel red cell flux cause unstable oxygen levels in tumor interstitium, extravascular oxygenation of R3230Ac mammary tumors grown in skin-fold window chambers was measured using recessed tip polarographic microelectrodes. Red cell fluxes in microvessels surrounding pO2 measurement locations were measured using fluorescently labeled red cells. Temporal pO2 instability was observed in all experiments. Median pO2 was inversely related to radial distance from microvessels. Transient fluctuations above and below 10 mm Hg were consistently seen, except in one experiment near the oxygen diffusion distance limit (140 microm) where pO2 fluctuations were <2 mm Hg and median pO2 was <5 mm Hg. Vascular stasis was not seen in these experiments. These results show that fluctuations in red cell flux, as opposed to vascular stasis, can cause temporal variations in pO2 that extend from perivascular regions to the maximum oxygen diffusion distance.

A conserved stem-loop structure in the 3' untranslated region of procyclin mRNAs regulates expression in Trypanosoma brucei

African trypanosomes that cycle between mammalian hosts and the tsetse fly vector must be poised to survive in different environments. The control of stage-specific gene expression is undoubtedly one of the keys to successful adaptation, but no regulatory elements have been defined to date. Procyclins (also known as procyclic acidic repetitive proteins) are specifically expressed on the surface of procyclic and epimastigote forms in the fly. Procyclin genes are already transcribed in bloodstream forms, but stable mRNA, and later the protein, are first detected when the parasites begin to differentiate into procyclic forms. We have now identified a region of 16 bases that forms part of a predicted stem-loop structure in the 3' untranslated regions of different procyclin mRNAs; both the sequence and the secondary structure of this 16-mer appear to be required for efficient translation of a reporter gene in procyclic forms. The level of steady-state mRNA, its polyadenylylation, and its distribution in the cell are all unaffected by the presence or absence of this element. Deletion of the 16-mer alone reduces expression more than removal or reversal of the entire 3' untranslated region and flanking region, suggesting that there are additional negative regulatory elements in the same 3' untranslated region.

Process development and optimisation of lactic acid purification using electrodialysis

Cell free sodium lactate solutions were subjected to purification based on mono- and bi-polar electrodialysis. Lactate concentration in the product stream increased to a maximum of 15% during mono-polar electrodialysis. Stack energy consumption averaged 0.6 kW h kg(-1) lactate transported at current efficiencies in the 90% range. Under optimum feed concentration (125 g l(-1)) and process conditions (auto-current mode with conductivity setpoints of minimum 5 and maximum 40 mS cm(-1)), lactate flux reached 300 g m(-2) h(-1) and water flux were low for mono-polar electrodialysis averaging 0.3 kg H(2)O per M lactate transported. Glucose in the concentrate stream solutions was reduced to < 2 g l(-1). Acetate impurities enriched from about 0.5 g l(-1) in the feed stream to 1.5 g l(-1) in the concentrate stream solutions. After mono-polar electrodialysis, the concentrated sodium lactate solutions were further purified using bi-polar electrodialysis. Water transport during bi-polar electrodialysis reached figures of 0.070 - 0.222 kg H(2)O per M lactate. Free lactic acid concentration reached 16% with lactate flux of up to 300 g m(-2) h(-1). Stack energy consumption ranged from 0.6 to 1 kW h per kg lactate. Under optimised process conditions current efficiency during bi-polar electrodialysis was consistently around 90%. Glucose was further reduced from 2 to <1 g l(-1) in the free lactic acid solution. Acetic acid impurity remained at around 1 g l(-1). Significant reduction in colour and minerals in the product streams was observed during electrodialysis purification.

Sequential temporal replication of DNA in Physarum polycephalum

Images

Control of polyadenylation and alternative splicing of transcripts from adjacent genes in a procyclin expression site: a dual role for polypyrimidine tracts in trypanosomes?

The procyclin-associated genes (PAGs) of Trypanosoma brucei are located downstream of tandemly repeated procyclin genes and belong to the same alpha-amanitin-resistant polycistronic transcription units. In procyclic form trypanosomes the PAG 1 pre-mRNA is alternatively spliced to give rise to three transcripts of 2.7 kb, 1.8 kb and 1.3 kb. The two larger transcripts contain additional short open reading frames (ORFs) upstream of the major ORF. Trans-splicing to generate these transcripts occurs downstream of three different polypyrimidine tracts. A minor population of procyclin mRNAs is also generated by alternative splicing at a polypyrimidine tract that begins 524 bp upstream of the major splice acceptor site of the procyclin beta-gene. The same polypyrimidine tract is also required for accurate polyadenylation of mRNAs from the upstream procyclin alpha-gene (1). Alternatively polyadenylated forms of PAG 1 mRNAs can also be detected. All polyadenylation sites are found at a similar distance upstream of splice-acceptor sites, in each case with a polypyrimidine tract between them. Our results point to a dual role for polypyrimidine tracts in the maturation of trypanosome mRNAs.

In vivo BOLD contrast MRI mapping of subcutaneous vascular function and maturation: validation by intravital microscopy

Bold contrast MRI was applied for mapping vascular maturation in tumor- and wound-induced skin angiogenesis using the response of mature vessels to hypercapnia (inhalation of air vs. air 5% CO(2)) and the response of all vessels to hyperoxia (air 5% CO(2) vs. oxygen 5% CO(2) (carbogen)). MRI signal enhancement with hypercapnia was reduced in centered vs. linear phase encoding, suggesting increased blood flow. However, intravital microscopy demonstrated constriction of arterioles and reduced flux and density of red blood cells in mature capillaries with hypercapnia, with no change in the diameter of wound-induced neovasculature. The discrepancy in flow between MRI and intravital microscopy is consistent with increased plasma flow and reduced hematocrit. Hyperoxia resulted in increased blood oxygenation and constriction of all vessels. These results provide a hemodynamic explanation for the selective registration of MRI response to hypercapnia with mature vessels and the response to hyperoxia with total vascular function.

Eimeria maxima gametocyte antigens: potential use in a subunit maternal vaccine against coccidiosis in chickens

Affinity-purified gametocyte antigens (APGA) from Eimeria maxima, emulsified in Freund's adjuvant, were injected intramuscularly into breeding hens on two or three occasions. As a result, progeny of the immunized hens were partially immune to infection with E. maxima, Eimeria tenella and Eimeria acervulina (with a reduction in total oocyst output of 45-63% as compared with progeny of untreated hens). Thus, APGA in Freund's adjuvant appears to have great potential as part of a maternally applied vaccine against coccidiosis. The ability of APGA to induce partial cross-species protection is most probably due to the existence of conserved epitopes in the different species as indicated by comparative Western blots of E. maxima and E. tenella. Surprisingly, Freund's adjuvant by itself also induced significant levels of maternal immunity to coccidiosis (with a 12-35% reduction in oocyst output in the progeny). In contrast to the purified antigens or Freund's alone, crude extracts from gametocytes as well as other developmental stages, induced little if any significant maternal immunity despite provoking the production of large amounts of parasite-specific IgG, including antibodies to APGA. This result indicates that a successful maternal vaccine against coccidiosis requires, in addition to good recognition of protective antigens, the exclusion of irrelevant antigens from the vaccine preparation.

A novel three-component one-pot pyrimidine synthesis based upon a coupling-isomerization sequence

[reaction: see text] 2,4,6-Tri(hetero)aryl-substituted pyrimidines can be readily synthesized in a three-component one-pot process based upon a coupling-isomerization sequence of an electron-poor (hetero)aryl halide and a terminal propargyl alcohol subsequently followed by a cyclocondensation with amidinium salts.

Teledermoscopy--results of a multicentre study on 43 pigmented skin lesions

We performed a multicentre study to evaluate the agreement between the direct clinical diagnosis and the telediagnosis of 43 cutaneous pigmented lesions. Digital clinical and dermoscopic images of the 43 pigmented skin lesions (11 melanomas, 23 melanocytic naevi, three basal cell carcinomas, three lentigines, two seborrhoeic keratoses and one angiokeratoma) were sent by email to 11 colleagues (six dermatologists, two residents in dermatology, one oncologist, one specialist in internal medicine and one general practitioner) in 10 centres. These 11 colleagues had different degrees of experience in dermoscopy. With histopathology as the gold standard, an average of 85% of the telediagnoses were correct, with results varying from 77% to 95%, whereas face-to-face diagnosis by an expert dermatologist was correct in 91% of cases. The kappa value for all participants ranged from 0.35 to 0.87. The results confirm that teledermoscopy can be a reliable technique for the diagnosis of pigmented skin lesions but one that will depend on the expertise of the observer.

Is periodontal disease the primary cause of tooth extraction in adults?

Preventing caries and reducing out-of-pocket costs for dental services are two strategies proposed by the authors to lower the rate of tooth extractions. Efforts also must be made to change public values about the importance of restoring rather than extracting carious teeth.

Tumor-dependent kinetics of partial pressure of oxygen fluctuations during air and oxygen breathing

The primary purpose of this study was to examine the kinetics of partial pressure of oxygen (pO2) fluctuations in fibrosarcoma (FSA) and 9L tumors under air and O2 breathing conditions. The overall hypothesis was that key factors relating to oxygen tension fluctuations would vary between the two tumor types and as a function of the oxygen content of the breathing gas. To assist in the interpretation of the temporal data, spatial pO2 distributions were measured in 10 FSA and 8 9L tumors transplanted into the subcutis of the hind leg of Nembutal-anesthetized (50 mg/kg) Fischer 344 rats. Recessed-tip oxygen microelectrodes were inserted into the tumor, and linear pO2 measurements were recorded in 50-microm steps along a 3-mm path, and blood pressure was simultaneously measured via femoral arterial access. Additionally, pO2 was measured at a single location for 90 to 120 minutes in FSA (n=11) or 9L tumors (n=12). Rats were switched from air to 100% O2 breathing after 45 minutes. Temporal pO2 records were evaluated for their potential radiobiological significance by assessing the number of times they crossed a 10-mm-Hg threshold. In addition, the data were subjected to Fourier analysis for air and O2 breathing. FSA and 9L tumors had spatial median pO2 measurements of 4 and 1 mm Hg, respectively. 9L had more low pO2 measurements < or =2.5 mm Hg than did FSA, whereas between 2.5 and 10 mm Hg this pattern was reversed. Pimonidazole staining patterns in FSA and 9L tumors supported these results. Temporal pO2 instability was observed in all experiments during air and O2 breathing. Threshold analyses indicated that the 10 mm Hg threshold was crossed 2 to 5 times per hour, independent of tumor type. However, the magnitude of 9L pO2 fluctuations was approximately eight times greater than FSA fluctuations, as assessed with Fourier transform analysis (Wilcoxon, P < 0.005). O2 breathing significantly increased median pO2 in FSA from 3 to 8 mm Hg (P < 0.005) and caused a significant increase in frequency and magnitude of pO2 fluctuations. One hundred percent O2 breathing had no effect on 9L tumor pO2, and it decreased the magnitude of pO2 fluctuations with borderline significance. These results show that these two tumors differ significantly with respect to spatial and temporal oxygenation conditions under air and O2 breathing. Fluctuations of pO2 of the type reported herein are predicted to significantly affect radiotherapy response and could be a source for genetic instability, increased angiogenesis, and metastases.

The replication of ribosomal DNA in Physarum polycephalum

The DNA coding for ribosomal RNA in Physarum polycephalum exists as a collection of extra-chromosomal molecules of molecular weight 37 X 10(6). We have investigated the replication of rDNA, with the following results. (a) Replication of rDNA is unscheduled. This means that molecules that are replicated at any particular time in one mitotic cycle have an equal probability of replicating again in each time interval in the subsequent cycle. Similarly, in a single cycle, some molecules replicate more than once, and some not at all. (b) Replication forks appear to move bidirectionally from points 45% or 33% from one end of the DNA. Replicating molecules observed by electron microscopy are all linear.

Tubulin genes of Trypanosoma brucei: a tightly clustered family of alternating genes

African trypanosomes are the causative agents of many medically and economically important diseases of man and domestic animals. The cell body of these blood-dwelling protozoa is enveloped with a dense layer of pellicular microtubules, which confer both motility and mechanical stability on these cells; microtubules are also important components of the flagellum. The major structural components of the microtubuli are two related proteins, alpha- and beta-tubulin. We have analyzed the genomic organization of alpha- and beta-tubulin genes in Trypanosoma brucei. In this organism, the majority of these genes are arranged in a single, tightly packed cluster of alternating alpha- and beta-tubulin genes with a basic repeat length together of 3.6 kilobase pairs. A genomic library of T. brucei was constructed by using the phage vector A 1059, and recombinant phages carrying tubulin genes were isolated by screening the library with heterologous chicken tubulin cDNA probes. The results of restriction endonuclease and hybridization analysis of DNA isolated from recombinant phages, and subcloned fragments thereof, were compatible with the restriction maps derived from digestion and Southern blot hybridization of genomic DNA.