Functional roles of carboxylate residues comprising the DNA polymerase active site triad of Ty3 reverse transcriptase

Aspartic acid residues comprising the -D-(aa)(n)-Y-L-D-D- DNA polymerase active site triad of reverse transcriptase from the Saccharomyces cerevisiae long terminal repeat-retrotransposon Ty3 (Asp151, Asp213 and Asp214) were evaluated via site-directed mutagenesis. An Asp151→Glu substitution showed a dramatic decrease in catalytic efficiency and a severe translocation defect following initiation of DNA synthesis. In contrast, enzymes harboring the equivalent alteration at Asp213 and Asp214 retained DNA polymerase activity. Asp151→Asn and Asp213→Asn substitutions eliminated both polymerase activities. However, while Asp214 of the triad could be replaced by either Asn or Glu, introducing Gln seriously affected processivity. Mutants of the carboxylate triad at positions 151 and 213 also failed to catalyze pyrophosphorolysis. Finally, alterations to the DNA polymerase active site affected RNase H activity, suggesting a close spatial relationship between these N- and C-terminal catalytic centers. Taken together, our data reveal a critical role for Asp151 and Asp213 in catalysis. In contrast, the second carboxylate of the Y-L-D-D motif (Asp214) is not essential for catalysis, and possibly fulfills a structural role. Although Asp214 was most insensitive to substitution with respect to activity of the recombinant enzyme, all alterations at this position were lethal for Ty3 transposition

Constraints on cosmic-ray acceleration and transport from isotope observations

Observations from the Cosmic Ray Isotope Spectrometer (CRIS) on ACE have been used to derive constraints on the locations, physical conditions, and time scales for cosmic-ray acceleration and transport. The isotopic composition of Fe, Co, and Ni is very similar to that of solar system material, indicating that cosmic rays contain contributions from supernovae of both Type II and Type Ia. The electron-capture primary ^(59)Ni produced in supernovae has decayed, demonstrating that a time ≳10^5 yr elapses before acceleration of the bulk of the cosmic rays and showing that most of the accelerated material is derived from old stellar or interstellar material rather than from fresh supernova ejecta

Implications for Cosmic Ray Propagation from ACE Measurements of Radioactive Clock Isotope Abundances

Galactic cosmic rays (GCR) interact to produce secondary fragments as they pass through the interstellar medium (ISM). Abundances of the long-lived radioactive secondaries ^(10)Be, ^(26)Al, ^(36)Cl, and ^(54)Mn can be used to a derive the confinement time of cosmic rays in the galaxy. Abundances for these species have been measured recently using the Cosmic Ray Isotope Spectrometer (CRIS) aboard the Advanced Composition Explorer (ACE) spacecraft. To interpret this data we have modeled the production and propagation of the radioactive secondaries, taking into account recently published isotopic production cross-sections. Abundances for all species are consistent with a confinement time of π_(esc) ~22 x 10^6 years

Cosmic Ray Source Abundances and the Acceleration of Cosmic Rays

The galactic cosmic ray elemental source abundances display a fractionation that is possibly based on first ionization potential (FIP) or volatility. A few elements break the general correlation of FIP and volatility and the abundances of these may help to distinguish between models for the origin of the cosmic ray source material. Data from the Cosmic Ray Isotope Spectrometer instrument on NASA’s Advanced Composition Explorer spacecraft were used to derive source abundances for several of these elements (Na, Cu, Zn, Ga, Ge). Three (Na, Cu, Ge) show depletions which could be consistent with a volatility-based source fractionation model

Time Variations of the Modulation of Anomalous and Galactic Cosmic Rays

Between the launch of the Advanced Composition Explorer (ACE) in 1997 and the end of 1999, the intensities of galactic cosmic rays at 1 AU have dropped almost a factor of 2, and the anomalous cosmic rays have decreased by an even larger amount. The large collecting power of the Cosmic Ray Isotope Spectrometer (CRIS) and the Solar Isotope Spectrometer (SIS) instruments on ACE allow us to investigate the changing modulation on short time scales and at different rigidities. Using anomalous cosmic ray (ACR) and galactic cosmic ray (OCR) intensities of He, C, O, Ne, Si, S, and Fe, and energies from ~ 6 MeV/nucleon to ~ 460 MeV/nucleon, we examine the differences between the short term and long term effects. We observe the expected correlation of these intensities with neutron monitor data, but see little correlation of OCR and ACR intensities with the locally measured magnetic field

The Time Delay between Nucleosynthesis and Acceleration Based on ACE Measurements of Primary Electron-Capture Nuclides

Supernovae should produce the radioactive nuclide ^(59)Ni, and in the ejecta of the explosions these particles will decay by electron capture with a halflife of 7.6 x 10^5 yr to produce ^(59)Co. However, if the ^(59)Ni nuclei are accelerated to cosmic-ray energies on a time scale short compared to this halflife, they are stripped of their electrons and decay is prevented. Thus the abundances of ^(59)Ni and ^(59)Co can be used to determine whether the time between nucleosynthesis and cosmic-ray acceleration is short or long compared to the 59Ni halflife (Soutoul, Casse, & Juliusson 1978). We have used the Cosmic Ray Isotope Spectrometer (CRIS) on the Advanced Composition Explorer (ACE) to measure the abundances of 59Ni and 59Co in galactic cosmic rays, and find that the data are consistent with complete decay of 59Ni indicating a time delay 2 10^5 yr. We present the observations and discuss their significance for models of cosmic ray origin and acceleration

Measurements of the Isotopic Abundances of Galactic Cosmic Rays and Their Implications for Cosmic Ray Origin

We present measurements of the nickel and cobalt isotopes from the Cosmic Ray Isotope Spectrometer (CRJS) which was launched in August, 1997 aboard the NASA Advanced Composition Explorer (ACE). The objectives of CRJS are to measure the isotopic abundances and energy spectra of galactic cosmic rays ~GCRs) with 3 ≤ Z ≤ 30. We find that for these nuclei the measured ^5Ni/^(60)Ni and ^(59)Co/^(60)Ni ratios imply a time delay between nucleosynthesis and acceleration to cosmic ray energies of > 10^5 y

Cosmic Ray Source Abundances for 29≤Z≤34

The Cosmic Ray Isotope Spectrometer (CRIS) instrument on board the Advanced Composition Explorer (ACE) spacecraft is making new measurements of nuclides just beyond the iron-nickel peak in the galactic cosmic rays. Isotopes of copper and zinc have been resolved for the first time. Elemental abundances for nuclei with 29≤Z≤34 are reported with good separation between species. Several of these elements are useful for studying fractionation processes which may depend on the first ionization potential or volatility. Source abundances are estimated using a prior propagation calculation and their potential for distinguishing between source models are discussed

Radioactive Clock Isotope Abundance Measurements from the CRIS Experiment aboard the ACE Spacecraft

Radioactive cosmic ray nuclei produced by nuclear interactions during cosmic ray propagation through the galaxy can be used to study the mean interstellar gas density in the propagation volume and the time scales associated with the propagation process. The Cosmic Ray Isotope Spectrometer (CRIS) aboard the Advanced Composition Explorer (ACE) has made high-resolution abundance measurements of the beta-decay secondary isotopes ^(10)Be, ^(26)Al, ^(36)Cl, and ^(54)Mn over the energy range 70-400 MeV/nuc. The large geometrical factor of CRIS (~250 cm^2sr) and the 20 months of data collection at near solar minimum conditions have made it possible since launch in August, 1997 to accumulate data samples considerably larger than previous missions. The isotopic abundances derived from these data are presented and compared with previous measurements