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G Porter

Publications and source records attributed to G Porter.

At least 37 records · Page 2Linked to original sources
PubMed1993

Rate of oxidation of P680 in isolated photosystem 2 reaction centers monitored by loss of chlorophyll stimulated emission.

We have continued our studies of the primary photochemistry of isolated photosystem 2 reaction centers using femtosecond transient absorption spectroscopy. Experiments were performed over a wide range of excitation and probe wavelengths, using several data collection time scales. This has enabled us to resolve five different lifetimes ranging between 100 fs and 200 ps plus a nanosecond component. We demonstrate here and elsewhere [e.g., Durrant, J.R., Hastings, G., Joseph, D. M., Barber, J., Porter, G., & Klug, D. R. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 11632-11636] that the kinetic spectra associated with all but two of these lifetimes are clearly distinguishable. We have previously reported that a 21-ps lifetime is associated with pheophytin reduction [Hastings, G., Durrant, J. R., Hong, Q., Barber, J., Porter, G., & Klug, D. R. (1992) Biochemistry 31, 7638-7647]. In this paper, we show that it is possible to spectrally and temporally resolve stimulated emission from PS2 reaction centers with great accuracy and that this stimulated emission is largely unaffected by those kinetic components which are faster than 21 ps. The observation of a distinct stimulated emission band allows us to distinguish charge-separated states from chlorin singlet states. In this way, we are able to show that the proportion of charge-separated states prior to the 21-ps component is between 0% and 25%. We also show that the shape of the spectrum which we obtain for the state P680+Ph- is essentially invariant between 100 ps and 9 ns, and is the same as that previously reported for P680+Ph- by other researchers.(ABSTRACT TRUNCATED AT 250 WORDS)

Chlorophyll↗
PubMed1993

Recognition by HLA-A2-restricted cytotoxic T lymphocytes of endogenously generated and exogenously provided synthetic peptide analogues of the influenza A virus matrix protein.

Experiments were carried out to determine whether complexes between MHC class I molecules and synthetic peptides are representative of those formed under more physiologically relevant conditions, with peptides derived intracellularly from processed antigens. Lysis of cells sensitized with exogenously provided and endogenously generated peptide analogues of the optimal nonameric peptide 58-66 (GILGFVFTL; derived from the influenza virus matrix protein) was compared. Endogenous loading was accomplished by expressing minigene DNA coding for alanine-substituted analogues of peptide 58-66 in HLA-A2-positive cells. Susceptibility to lysis by HLA-A2-restricted, peptide-specific cytotoxic lymphocytes was compared with lysis of cells sensitized with the same synthetic peptides. Although results were quite comparable, differences were observed. The endogenously presented analogues 58-66L60A, G61A, T65A, and L66A were recognized more efficiently than the corresponding exogenously presented analogues. This difference in recognition was most striking for peptide 58-66G61A. These results indicate the need for caution in using synthetic peptides in defining peptide binding motifs. Additional experiments with endogenously expressed analogues of 58-66 with substitutions other than alanine were carried out to define the interaction between this peptide and HLA-A2. Results are compatible with the interpretation that residues 58, 59, and 60 interact with pockets A, B, and D, respectively, in the HLA-A2 binding groove and that these interactions contribute to peptide binding.

Amino Acid Sequence↗
PubMed1993

A clinical and radiological study of back pain in rheumatoid arthritis.

Five hundred and three patients with RA were questioned about the symptom of back pain. Chronic back pain, lasting more than 3 months, occurred in 33 per cent of the group. A group of 100 back pain patients were studied in more detail using a structured questionnaire, clinical examination and radiology. Ninety-four of these patients had low back pain. Particular clinical patterns (such as that of the facet syndrome) were sought but no clear characteristics were found. Fifty-two lumbar spine X-rays were available from the RA population and these were compared to 52 age and sex matched X-rays from outpatients with chronic mechanical low back pain. Significant differences between these groups radiologically were a higher frequency of osteoporosis and a higher frequency of disc narrowing without associated osteophytes in the RA population. This study differs from previous reports which found other characteristic radiological features of RA of the lumbar spine (spondylolisthesis, facet erosions, and vertebral fracture), a discrepancy possibly resulting from the use of a control group having low back pain.

Adult↗
PubMed1992

Subpicosecond equilibration of excitation energy in isolated photosystem II reaction centers.

Photosystem II reaction centers have been studied by femtosecond transient absorption spectroscopy. We demonstrate that it is possible to achieve good photoselectivity between the primary electron donor P680 and the majority of the accessory chlorins. Energy transfer can be observed in both directions between P680 and these accessory chlorins depending on which is initially excited. After excitation of either P680 or the other chlorins, the excitation energy is observed to equilibrate between the majority of these pigments at a rate of 100 +/- 50 fs-1. This energy-transfer equilibration takes place before any electron-transfer reactions and must therefore be taken into account in studies of primary electron-transfer reactions in photosystem II. We also show further evidence that the initially excited P680 excited singlet state is delocalized over at least two chlorins and that this delocalization lasts for at least 200 fs.

Chloroplasts↗
PubMed1992

Observation of pheophytin reduction in photosystem two reaction centers using femtosecond transient absorption spectroscopy.

Photosystem two reaction centers have been studied using a sensitive femtosecond transient absorption spectrometer. Measurements were performed at 295 K using different excitation wavelengths and excitation intensities which are shown to avoid multiphoton absorption by the reaction centers. Analyses of results collected over a range of time scales and probe wavelengths allowed the resolution of two exponential components in addition to those previously reported [Durrant, J. R., Hastings, G., Hong, Q., Barber, J., Porter, G., & Klug, D. R. (1992) Chem. Phys. Lett. 188, 54-60], plus the long-lived radical pair itself. A 21-ps component was observed. The process(es) responsible for this component was (were) found to produce bleaching of a pheophytin ground-state absorption band at 545 nm and the simultaneous appearance of a pheophytin anion absorption band at 460 nm resulting in a transient spectrum which was that of the radical pair P680+Ph-. This component is assigned to the production of reduced pheophytin. A lower limit of 60% of the final pheophytin reduction was found to occur at this rate. Despite subtle differences in transient spectra, the lifetime and yield of this pheophytin reduction are essentially independent of excitation wavelength within the signal to noise limitations of these experiments. A long-lived species was also observed. This species is produced by those processes which result in the 21-ps component, and it has a spectrum which is found to be independent of excitation wavelength. This spectrum is characteristic of the primary radical pair state P680+Ph-. In addition, a 200-ps component was found which is tentatively assigned to a slow energy-transfer/trapping process. This component was absent if P680 was excited directly and is therefore not integral to primary radical pair formation. Overall, it is concluded that the rate of pheophytin reduction is limited to (21 ps)-1, even when P680 is directly excited.

Fabaceae↗
PubMed1992

Occult posttraumatic avascular necrosis of hip revealed by MRI.

Three hips with avascular necrosis in two patients had associated traumatic lesions noted on magnetic resonance imaging. Location of stress fracture in two femoral necks and acetabular contusion development prior to avascular necrosis favors these processes as being etiologic. These cases are particularly noteworthy in that nondisplaced fractures or other musculoskeletal trauma not visible on plain films has rarely been associated with subsequent development of avascular necrosis.

Acetabulum↗
PubMed1992

The acute contrast enema in suspected large bowel obstruction: value and technique.

In order to determine the value of the acute contrast enema (ACE) as compared to the plain abdominal film for the diagnosis of colonic obstruction and to determine the optimal technique for performing this examination, we reviewed 140 cases performed over a 4 year period. The study shows that when provided with the case history and the plain abdominal films of patients referred for an ACE the diagnosis of colonic obstruction can be made with a sensitivity of 84% and a specificity of 72%, whereas the ACE has a sensitivity of 96% and a specificity of 98%. The ACE resulted in two false negatives, one false positive and one technical failure, analysis of which has enabled an optimal technique for the performance of this examination to be recommended.

Colon↗
PubMed1992

Fatal destructive cervical spondyloarthropathy in two patients on long-term dialysis.

Two patients with fatal cervical cord compressive myelopathy are described, both of whom had been on dialysis for more than 15 years. Destructive changes were noted in mid and upper cervical regions, with soft tissue mass in the atlanto-occipital region in one patient. Clinical and radiographic findings suggest both amyloid and hyperparathyroidism as possible etiologies for these destructive spinal changes. Clinicians should be aware that the full picture of quadriparesis may be associated with destructive spondyloarthropathy (DSA) in long-term dialysis patients.

Aged↗
PubMed1991

The minimum peptide epitope from the influenza virus matrix protein. Extra and intracellular loading of HLA-A2.

Influenza virus matrix protein-derived peptides were synthesized based on the amino acid motifs for HLA-A2 bound self peptides. Among these peptides a nonamer (amino acids 58 through 66: G I L G F V F T L) was found to be 100 to 1000 times more effective than the commonly used peptide 57-68 (K G I L G F V F T L T V) in sensitizing HLA-A2+ target cells to lysis by influenza virus specific cytotoxic T lymphocytes. The sensitizing activity of the 12-mer 57-68 was not due to contamination with shorter and more active peptides. Intracellular expression of peptide 58-66 (mediated by a stable expression plasmid with DNA coding for this peptide) also sensitized HLA-A2+ cells to lysis. Peptide 58-66 stimulated human PBMC to generate CTL that recognized peptides 58-66 and 57-68 in association with HLA-A2.

Amino Acid Sequence↗
PubMed1991

Observation of multiple radical pair states in photosystem 2 reaction centers.

Charge recombination of the primary radical pair in D1/D2 reaction centers from photosystem 2 has been studied by time-resolved fluorescence and absorption spectroscopy. The kinetics of the primary radical pair are multiexponential and exhibit at least two lifetimes of 20 and 52 ns. In addition, a third lifetime of approximately 500 ps also appears to be present. These multiexponential charge-recombination kinetics reflect either different conformational states of D1/D2 reaction centers, with the different conformers exhibiting different radical pair lifetimes, or relaxations in the free energy of the radical pair state. Whichever model is invoked, the free energies of formation of the different radical pair states exhibit a linear temperature dependence from 100 to 220 K, indicating that they are dominated by entropy with negligible enthalpy contributions. These results are in agreement with previous determinations of the thermodynamics that govern primary charge separation in both D1/D2 reaction centers [Booth, P.J., Crystall, B., Giorgi, L. B., Barber, J., Klug, D.R., & Porter, G. (1990) Biochim. Biophys. Acta 1016, 141-152] and reaction centers of purple bacteria [Woodbury, N.W.T., & Parson, W.W. (1984) Biochim. Biophys. Acta 767, 345-361]. It is possible that these observations reflect structural changes that accompanying primary charge separation and assist in stabilization of the radical pair state thus optimizing the efficiency of primary electron transfer.

Cytochrome b Group↗
PubMed1991

Soluble HLA-A2.1 restricted peptides that are recognized by influenza virus specific cytotoxic T lymphocytes.

The influenza A virus matrix protein derived peptide with amino acids 57-68 (Lys-Gly-Ileu-Leu-Gly-Phe-Val-Phe-Thr-Leu-Thr-Val) is recognized by influenza virus HLA-A2 restricted CTL. Because of the large number of hydrophobic residues this peptide is very insoluble. Substitution with a number of polar amino acids resulted in a soluble peptide (Lys-Lys-Ala-Leu-Gly-Phe-Val-Phe-Thr-Leu-Asp-Lys) that was very effective in sensitizing HLA-A2 positive target cells. Further substitution of threonine in position 65 with lysine resulted in a soluble antagonist peptide that inhibited sensitization. Both agonist and antagonist peptides retained 20% of their biological activity when tyrosine was added at the N terminus. Soluble radio-iodinated peptides can now be prepared that will be useful reagents to study the interaction of peptides and class I molecules.

Amino Acid Sequence↗
PubMed1991

Patient-controlled analgesia--an anaesthetist's view.

The perception of pain is highly subjective, being influenced by physical, emotional and cultural factors. The inadequate management of postoperative pain has been widely documented over the last 40 years.

Analgesia, Patient-Controlled↗
PubMed1990

How easy is it to contact the duty medical doctor responsible for acute admissions?

OBJECTIVE: To ascertain ease or difficulty of contacting duty junior doctors responsible for acute medical admissions by telephone. DESIGN: Telephone survey of hospitals in six health regions in England and Wales. SETTING: 70 Randomly selected hospitals, 15 of which were excluded because of non-acceptance of acute medical admissions. PARTICIPANTS: 71 Duty doctors (duty house physicians, senior house officers, or registrars responsible for acute medical admissions) in 48 hospitals; seven duty doctors in seven hospitals were excluded (four declined to participate and three required a written explanation of the survey). 67 Doctors gave full information to all questions. MAIN OUTCOME MEASURES: Time taken for hospital switchboards and duty doctors to reply to telephone call, diagnoses of patients recently admitted, and on call rotas and hours of sleep of duty doctors. RESULTS: Hospital switchboards responded within 30 seconds in 87 (74%) calls, and in 76 calls (64%) the duty doctor requested was contacted within a further two minutes. Chest pain, possibly due to myocardial infarction, was the most common reason for acute medical admissions. Nearly half (48%) of the duty doctors in larger hospitals reported having 4-5 hours sleep or less on their nights on call. Most (30) were on a one in three rota; two were on a one in two rota. CONCLUSIONS: Despite impressions to the contrary contacting the duty medical team by telephone seemed fairly easy. Although most junior doctors were on a rota of one in three or better, insufficient recognition may be given to their deprivation of sleep during nights on duty.

Communication↗
PubMed1990

Comparison of the D1/D2/cytochrome b559 reaction centre complex of photosystem two isolated by two different methods.

Photosystem 2 reaction centre complexes prepared either by solubilisation with Triton X-100 and subsequent exchange into dodecyl maltoside or by a procedure involving a combination of dodecyl maltoside and LiClO4, were characterised in terms of chlorophyll a, pheophytin a, beta-carotene and cytochrome b559 content. Time-resolved chlorophyll fluorescence decay kinetics were measured using both types of complexes. Our data show that the isolated photosystem two reaction centre complex contain, for two pheophytin a molecules, close to six chlorophyll a, two beta-carotene and one cytochrome b559. No major differences were observed in the composition or the kinetic characteristics measured in the samples prepared by the different procedures. Time-resolved fluorescence measurements indicate that more than 94% of the chlorophyll a in both preparations is coupled to the reaction centre complex.

Chlorophyll↗
PubMed1990

U1 small nuclear RNA from Schizosaccharomyces pombe has unique and conserved features and is encoded by an essential single-copy gene.

We have cloned, sequenced, and disrupted the gene encoding U1 small nuclear RNA (snRNA) in the fission yeast Schizosaccharomyces pombe. This RNA is close in size and exhibits a high degree of secondary structure homology to human U1 RNA. There exist two regions of extended primary sequence identity between S. pombe and human U1 RNAs; the first comprises nucleotides involved in hydrogen bonding to 5' splice junctions, and the second is a single-stranded region which, in the human snRNA, forms part of the A protein binding site. S. pombe U1 lacks two nucleotides just following the 5' cap structure which are present in all other U1 homologs examined to date, and the region which corresponds to the binding site for the human 70K protein (molecular weight of 55,000) is more divergent than in other organisms. A putative upstream transcription signal is conserved in sequence and location among all loci encoding spliceosomal snRNAs in S. pombe with the exception of U6. Disruption of the single-copy U1 gene, designated snu1, reveals that this RNA is indispensable for viability.

Base Sequence↗
PubMed1988

U2 small nuclear RNA is remarkably conserved between Schizosaccharomyces pombe and mammals.

We report the molecular cloning and sequencing of the most abundant trimethylguanosine-capped small nuclear RNA from the fission yeast Schizosaccharomyces pombe, a highly conserved homolog of mammalian U2 small nuclear RNA. This RNA is 186 nucleotides in length, just 2 nucleotides shorter than its human counterpart; this is in contrast to Saccharomyces cerevisiae U2, which is 1,175 nucleotides long. Moreover, the secondary structure of Schizosaccharomyces pombe U2 is virtually identical to that of mammalian U2, including the 3' half of the RNA, which shows limited primary sequence identity. Northern (RNA) blot analysis revealed that the size of this RNA is conserved not only in fission yeasts but in many organisms, including other ascomycetes.

Animals↗
PubMed1988

Identification of an essential Schizosaccharomyces pombe RNA homologous to the 7SL component of signal recognition particle.

We have cloned the gene encoding a novel small cytoplasmic RNA from the fission yeast Schizosaccharomyces pombe. Four lines of evidence support the idea that this RNA is a homolog of the 7SL RNA component of mammalian signal recognition particle (SRP), which targets presecretory proteins to the endoplasmic reticulum membrane. First, it shares limited but significant primary sequence homology with previously identified 7SL RNAs and can be folded into a similar secondary structure. Second, it possesses the 5' triphosphate characteristic of unprocessed RNA polymerase III transcripts, and moreover, it is the only fission yeast RNA in this size range with such a terminus. Third, its behavior in cell fractionation experiments suggests that it is part of a small ribonucleoprotein which forms salt-labile contacts with larger structures. Fourth, the particle containing S. pombe 7SL RNA resembles mammalian SRP in both size (11S) and affinity for DEAE-Sepharose. Disruption of the single-copy gene, designated slr1+, reveals that the RNA is indispensable for growth in fission yeast. This result is not surprising, since secretion is an essential cellular process.

Base Sequence↗