No abstract

No abstract

No abstract

An ab initio linear-response approach based on the density-functional theory has been used to investigate the dynamics of bulk IuP and of the IuP(110) surface. The plane-wave pseudopotential method together vrith the slab supercell description for the surface have been employed in order to determine the complete bulk phonou spectrum, the atomic geometry of the IuP(110) surface, and the surface phonon dispersion in a consistent formalism. The calculated structural and dynamical properties of both the bulk and the surface compare very well vrith all available experimental data. Furthermore, we examine the inHuence of the structural details of InP(110) on the surface vibrational features. Our ab initio results indicate the fingerprints of the relaxation geometry in the surface phonon dispersion. I. INTR. ODU CTIONDensity-functional calculations based on the linearresponse formalism proposed by Baroni, Giannozzi, and Testa have proven to be a very efEcient method for determining the lattice dynamics in elemental and compound semiconductors.The calculated phonon dispersion curves compare very well with the spectra obtained from inelastic neutron difFraction and Raman scattering. A recent application of ab initio linear-response theory to GaAs(110) has shown that also for the surface of a binary semiconductor, the atomic equilibrium positions and the vibrations can be determined within the same formalism, completely consistent with the calculation of the structural and dynamical bulk properties without introducing any adjustable parameters or modifications. The agreement of the calculated surface phonon dispersion with all available experimental data from inelastic scattering of He atoms ' and high-resolution electronenergy-loss spectroscopy (HREELS) is comparable to the accuracy of the ab initio results for the bulk phonon spectra. Among all previous theoretical investigations on bulk InP and InP (110), first-principles calculations have been devoted only to the determination of the zone boundary frequencies of the longitudinal bulk phonons at the X point within the frozen-phonon approach and to the examination of the atomic geometry and the electronic states of the (110) surface. 's In the present paper, we have used the density-functional perturbation theory (DFPT) of Baroni and co-workersi 2 in the extension of Ref. 3, in order to perform an ab initio calculation of the complete vibrational spectra for bulk InP and for the InP(110) surface. An application of DFPT to the study of the structure and phase stability of Ga Inq P solid solutions was presented in Ref. 10. The bulk phonon dispersion of InP has been measured with high precision by inelastic neutron scattering and by Raman spectroscopy. ' With the exception of Ref. 7, most of the previous theoretical investigations on the lattice vibrations in InP are based on model calculations, the results being characterized by an underestimation of the frequency of the LA phonon near the I point and by diferent predictions for the dispersion of the optical phonons in the...

We have identified an mRNA element that is involved in the initial cleavage of the pufBALMX mRNA species in Rhodobacter capsulatus. This endoribonuclease recognition site, the first to be identified in a bacterial species other than Escherichia coli, shows strong similarities to mRNA sequences cleaved by the endoribonuclease E in E. coli. The presence of an RNase E-like enzyme in R. capsulatus is further supported by in vitro cleavage of E. coli transcripts by R. capsulatus extracts at sites attributed to RNase E and by the cross-reaction of a polypeptide from R. capsulatus with antisera against E. coli RNase E. Our data provide evidence that mRNAs are degraded in different bacterial species by enzymes with similar recognition sequences and activities. We present a model that attributes the segmental differences in stability of the polycistronic puf transcript to a specific distribution of mRNA decay-promoting and mRNA decay-impeding elements.