Three-body nature of N(*) and Δ(*) resonances from sequential decay chains

The Nπ^{0}π^{0} decays of positive-parity N^{*} and Δ^{*} resonances at about 2 GeV are studied at ELSA by photoproduction of two neutral pions off protons. The data reveal clear evidence for several intermediate resonances: Δ(1232), N(1520)3/2^{-}, and N(1680)5/2^{+}, with spin parities J^{P}=3/2^{+}, 3/2^{-}, and 5/2^{+}. The partial wave analysis (within the Bonn-Gatchina approach) identifies N(1440)1/2^{+} and the N(ππ)_{S wave} (abbreviated as Nσ here) as further isobars and assigns the final states to the formation of nucleon and Δ resonances and to nonresonant contributions. We observe the known Δ(1232)π decays of Δ(1910)1/2^{+}, Δ(1920)3/2^{+}, Δ(1905)5/2^{+}, Δ(1950)7/2^{+}, and of the corresponding spin-parity series in the nucleon sector, N(1880)1/2^{+}, N(1900)3/2^{+}, N(2000)5/2^{+}, and N(1990)7/2^{+}. For the nucleon resonances, these decay modes are reported here for the first time. Further new decay modes proceed via N(1440)1/2^{+}π, N(1520)3/2^{-}π, N(1680)5/2^{+}π, and Nσ. The latter decay modes are observed in the decay of N^{*} resonances and at most weakly in Δ^{*} decays. It is argued that these decay modes provide evidence for a 3-quark nature of N^{*} resonances rather than a quark-diquark structure.

Evidence for a parity doublet Delta(1920)P33 and Delta(1940)D33 from gammap-->ppi;{0}eta

Evidence is reported for the existence of a parity doublet of Delta resonances with total angular momentum J=3/2 from photoproduction of the ppi;{0}eta final state. The two parity partners Delta(1920)P33 and Delta(1940)D33 make significant contributions to the reaction. Cascades of resonances into Delta(1232)eta, N(1535)pi, and Na0(980) are clearly observed.

Quasifree photoproduction of eta mesons off the neutron

Quasifree photoproduction of eta mesons off nucleons bound in the deuteron has been measured with the CBELSA/TAPS detector for incident photon energies up to 2.5 GeV at the Bonn ELSA accelerator. The eta mesons have been detected in coincidence with recoil protons and recoil neutrons, which allows a detailed comparison of the quasifree n(gamma,eta)n and p(gamma,eta)p reactions. The excitation function for eta production off the neutron shows a pronounced bumplike structure at W=1.68 GeV (E{gamma} approximately 1 GeV), which is absent for the proton.

Modification of the omega-meson lifetime in nuclear matter

Information on hadron properties in the nuclear medium has been derived from the photoproduction of omega mesons on the nuclei C, Ca, Nb, and Pb using the Crystal Barrel/TAPS detector at the ELSA tagged photon facility in Bonn. The dependence of the omega-meson cross section on the nuclear mass number has been compared with three different types of models: a Glauber analysis, a Boltzmann-Uehling-Uhlenbeck analysis of the Giessen theory group, and a calculation by the Valencia theory group. In all three cases, the inelastic omega width is found to be 130-150 MeV/c(2) at normal nuclear matter density for an average 3-momentum of 1.1 GeV/c. In the rest frame of the omega meson, this inelastic omega width corresponds to a reduction of the omega lifetime by a factor approximately 30. For the first time, the momentum dependent omegaN cross section has been extracted from the experiment and is in the range of 70 mb.

Observation of in-medium modifications of the omega meson

The photoproduction of omega mesons on nuclei has been investigated using the Crystal Barrel/TAPS experiment at the ELSA tagged photon facility in Bonn. The aim is to study possible in-medium modifications of the omega meson via the reaction gamma + A --> omega + X --> pi(0)gamma + X('). Results obtained for Nb are compared to a reference measurement on a LH2 target. While for recoiling, long-lived mesons (pi(0), eta, and eta;(')), which decay outside of the nucleus, a difference in the line shape for the two data samples is not observed, we find a significant enhancement towards lower masses for omega mesons produced on the Nb target. For momenta less than 500 MeV/c an in-medium omega meson mass of M(medium) = [722(+4)(-4)(stat)+35-5(syst)] MeV/c(2) has been deduced at an estimated average nuclear density of 0.6rho(0).

Photoproduction of eta mesons off protons for 0.75 GeV<E(gamma)<3 GeV

Total and differential cross sections for the reaction p(gamma,eta)p have been measured for photon energies in the range from 750 MeV to 3 GeV. The low-energy data are dominated by the S11 wave which has two poles in the energy region below 2 GeV. Eleven nucleon resonances are observed in their decay into peta. At medium energies we find evidence for a new resonance N(2070)D15 with (M,Gamma)=(2068+/-22, 295+/-40) MeV. At gamma energies above 1.5 GeV, a strong peak in the forward direction develops, signaling the exchange of vector mesons in the t channel.

Neutral-pion photoproduction off protons in the energy range 0.3 GeV<E(gamma)<3 GeV

Single pi(0) photoproduction has been studied with the CB-ELSA experiment at Bonn using tagged photon energies between 0.3 and 3.0 GeV. The experimental setup covers a very large solid angle of approximately 98% of 4pi. Differential cross sections dsigma/dOmega have been measured. Complicated structures in the angular distributions indicate a variety of different resonances being produced in the s channel intermediate state gammap-->N(*)(Delta(*))-->ppi(0). A combined analysis including the data presented in this letter along with other data sets reveals contributions from known resonances and evidence for a new resonance N(2070)D15.

Critical base substitutions that affect the splicing and/or homing activities of the group I intron bi2 of yeast mitochondria

The second intron (bi2) of the cyt b gene from related Saccharomyces species has an extraordinarily conserved sequence and can have different functions in wild-type cells. The protein encoded by the S. cerevisiae intron functions as a maturase to promote intron splicing, while the homologous S. capensis intron encodes a bifunctional protein that acts both as a maturase and as a homing endonuclease (I-ScaI) promoting intron mobility. The protein encoded by intron bi2 belongs to a large gene family characterized by the presence of two conserved LAGLIDADG motifs (P1 and P2). In this study, we analysed a set of splicing-deficient mutants of the S. cerevisiae intron bi2 that carry non-directed mutations affecting the maturase activity, and a set of directed missense mutations introduced into the bifunctional protein encoded by the S. capensis intron. Analysis of these mutations has allowed identification of the residues in the conserved P1 and P2 motifs which are crucial for splicing and homing activities. Moreover, several mutations which are located in the C-terminal part of the protein have been found to affect both functions.

Purification and characterization of the DNA cleavage and recognition site of I-ScaI mitochondrial group I intron encoded endonuclease produced in Escherichia coli

The second intron in the mitochondrial cytb gene of Saccharomyces capensis, belonging to group I, encodes a 280 amino acid protein containing two LAGLIDADG motifs. Genetic and molecular studies have previously shown that this protein has a dual function in the wild-type strain. It acts as a specific homing endonuclease I- Sca I promoting intron mobility and as a maturase promoting intron splicing. Here we describe the synthesis of a universal code equivalent to the mitochondrial sequence coding for this protein and the in vitro characterization of I- Sca I endonuclease activity, using a truncated mutant form of the protein p28bi2 produced in Escherichia coli. We have also determined the cleavage pattern as well as the recognition site of p28bi2. It was found that p28bi2 generates a double-strand cleavage downstream from the intron insertion site with 4 nt long 3'-overhangs. Mutational analysis of the DNA target site shows that p28bi2 recognizes a 16-19 bp sequence from positions -11 to +8 with respect to the intron insertion site.

Replacement of two non-adjacent amino acids in the S.cerevisiae bi2 intron-encoded RNA maturase is sufficient to gain a homing-endonuclease activity

Two homologous group I introns, the second intron of the cyt b gene, from related Saccharomyces species differ in their mobility. The S.capensis intron is mobile and encodes the I-ScaI endonuclease promoting intron homing, whilst the homologous S.cerevisiae intron is not mobile, but functions as an RNA maturase promoting splicing. These two intron-encoded proteins differ by only four amino acid substitutions. Taking advantage of the remarkable similarity of the two intron open reading frames and using biolistic transformation of mitochondria, we show that the replacement of only two non-adjacent residues in the S.cerevisiae maturase carboxy-terminal sequence is sufficient to induce a homing-endonuclease activity without losing the splicing function. Also, we demonstrate that these two activities reside in the S.capensis bi2-encoded protein which functions in both splicing and intron mobility in the wild-type cells. These results provide new insight into our understanding of the activity and the evolution of group I intron-encoded proteins.

The S. cerevisiae nuclear gene SUV3 encoding a putative RNA helicase is necessary for the stability of mitochondrial transcripts containing multiple introns

The product of the nuclear gene SUV3 is implicated in a variety of post-transcriptional processes in yeast mitochondria. We have analysed the effect of SUV3 gene-disruption on the expression of intron-containing alleles of the mitochondrial cytb and cox1 genes. We have constructed several strains with mitochondrial genomes containing different combinations of cytb and cox1 introns, and associated these genomes with the disruption of SUV3. The resulting strains were tested for their respiratory competence and spectral cytochrome content. All the strains containing only two or three introns showed normal expression of cytb and cox1, whereas the strains containing more introns were unable to express the appropriate gene. The analysis of mitochondrial RNAs by Northern hybridisation showed that the loss of respiratory competence in the strains containing more introns is due to the decrease of mRNA level with no over-accumulation of high-molecular-weight precursors. However, the transcription of the genes was not affected. These results led us to the notion that SUV3 is required for the stability of intron-containing cytb and cox1 transcripts in a cumulative way, not dependent on any particular intron.

Two homologous introns from related Saccharomyces species differ in their mobility

We have studied gene conversion initiated by the ai3 intron of the Saccharomyces cerevisiae mitochondrial (mt) COXI gene and its homologous intron (S.cap.ai1) from Saccharomyces capensis. The approach used involved the measurement of intron transmission amongst the progeny of crosses between constructed recipient and donor strains. We found that the S. cerevisiae ai3 intron is extremely active as a donor in gene conversion, whereas its homologous S. capensis intron is not. We have established the sequence of S.cap.ai1 and compared its open reading frame (ORF) with that of I-SceIII encoded by the homologous S. cerevisiae intron. The two protein-coding intron sequences are almost identical, except that the S. capensis ORF contains an in-frame stop codon. This finding provides a strong indication that the 3' part of the S. cerevisiae intron ORF encoding I-SceIII (which should not be translated in the S. capensis intron) must be critical for function of mtDNA endonucleases to mediate intron mobility.