AP endonuclease paralogues with distinct activities in DNA repair and bacterial pathogenesis

Oxidative stress is a principal cause of DNA damage, and mechanisms to repair this damage are among the most highly conserved of biological processes. Oxidative stress is also used by phagocytes to attack bacterial pathogens in defence of the host. We have identified and characterised two apurinic/apyrimidinic (AP) endonuclease paralogues in the human pathogen Neisseria meningitidis. The presence of multiple versions of DNA repair enzymes in a single organism is usually thought to reflect redundancy in activities that are essential for cellular viability. We demonstrate here that these two AP endonuclease paralogues have distinct activities in DNA repair: one is a typical Neisserial AP endonuclease (NApe), whereas the other is a specialised 3'-phosphodiesterase Neisserial exonuclease (NExo). The lack of AP endonuclease activity of NExo is shown to be attributable to the presence of a histidine side chain, blocking the abasic ribose-binding site. Both enzymes are necessary for survival of N. meningitidis under oxidative stress and during bloodstream infection. The novel functional pairing of NExo and NApe is widespread among bacteria and appears to have evolved independently on several occasions.     

Kinetic analysis of the interaction of guanine nucleotides with eukaryotic translation initiation factor eIF5B

Eukaryotic translation initiation factor eIF5B is a ribosome-dependent GTPase that is responsible for the final step in initiation, which involves the displacement of initiation factors from the 40S ribosomal subunit in initiation complexes and its joining with the 60S subunit. Hydrolysis of eIF5B-bound GTP is not required for its function in subunit joining but is necessary for the subsequent release of eIF5B from assembled 80S ribosomes. Here we investigated the kinetics of guanine nucleotide binding to eIF5B by a fluorescent stopped-flow technique using fluorescent mant derivatives of GTP and GDP and of the GTP analogues GTPgammaS and GMPPNP. The affinity of eIF5B for mant-GTP (Kd approximately 14-18 microM) was approximately 7-fold less than for mant-GDP (Kd approximately 2.3 microM), and both guanine nucleotides dissociated rapidly from eIF5B (k-1mant-GTP approximately 22-28 s-1, k-1mant-GDP approximately 10-14 s-1). These properties of eIF5B suggest a rapid spontaneous GTP/GDP exchange on eIF5B and are therefore consistent with it having no requirement for a special guanine nucleotide exchange factor. The affinity of eIF5B for mant-GTPgammaS was about 2 times lower (Kd approximately 6.9 microM) and for mant-GMPPNP 1.5 times higher (Kd approximately 25.7 microM) than for mant-GTP, indicating that eIF5B tolerates modifications of the triphosphate moiety well.     

Common conformational changes induced in type 2 picornavirus IRESs by cognate trans-acting factors

Type 2 internal ribosomal entry sites (IRESs) of encephalomyocarditis virus (EMCV), foot-and-mouth disease virus (FMDV) and other picornaviruses comprise five major domains H-L. Initiation of translation on these IRESs begins with specific binding of the central domain of initiation factor, eIF4G to the J-K domains, which is stimulated by eIF4A. eIF4G/eIF4A then restructure the region of ribosomal attachment on the IRES and promote recruitment of ribosomal 43S pre-initiation complexes. In addition to canonical translation factors, type 2 IRESs also require IRES trans-acting factors (ITAFs) that are hypothesized to stabilize the optimal IRES conformation that supports efficient ribosomal recruitment: the EMCV IRES is stimulated by pyrimidine tract binding protein (PTB), whereas the FMDV IRES requires PTB and ITAF(45). To test this hypothesis, we assessed the effect of ITAFs on the conformations of EMCV and FMDV IRESs by comparing their influence on hydroxyl radical cleavage of these IRESs from the central domain of eIF4G. The observed changes in cleavage patterns suggest that cognate ITAFs promote similar conformational changes that are consistent with adoption by the IRESs of comparable, more compact structures, in which domain J undergoes local conformational changes and is brought into closer proximity to the base of domain I.     

Investigation of the Origin and Spread of a Mammalian Transposable Element Based on Current Sequence Diversity

Almost half the human genome consists of mobile DNA elements, and their analysis is a vital part of understanding the human genome as a whole. Many of these elements are ancient and have persisted in the genome for tens or hundreds of millions of years, providing a window into the evolution of modern mammals. The Golem family have been used as model transposons to highlight computational analyses which can be used to investigate these elements, particularly the use of molecular dating with large transposon families. Whole-genome searches found Golem sequences in 20 mammalian species. Golem A and B subsequences were only found in primates and squirrel. Interestingly, the full-length Golem, found as a few copies in many mammalian genomes, was found abundantly in horse. A phylogenetic profile suggested that Golem originated after the eutherian-metatherian divergence and that the A and B subfamilies originated at a much later date. Molecular dating based on sequence diversity suggests an early age, of 175 Mya, for the origin of the family and that the A and B lineages originated much earlier than expected from their current taxonomic distribution and have subsequently been lost in some lineages. Using publically available data, it is possible to investigate the evolutionary history of transposon families. Determining in which organisms a transposon can be found is often used to date the origin and expansion of the families. However, in this analysis, molecular dating, commonly used for determining the age of gene sequences, has been used, reducing the likelihood of errors from deleted lineages.     

Initiation on the divergent Type I cadicivirus IRES: factor requirements and interactions with the translation apparatus

Cadicivirus (CDV) is unique amongst picornaviruses in having a dicistronic genome with internal ribosomal entry sites (IRESs) preceding both open reading frames. Here, we investigated initiation on the 5′-terminal IRES. We report that the 982-nt long 5′UTR comprises 12 domains (d1-d12), five of which (d8-d12, nts 341–950) constitute a divergent Type I IRES. It comprises central elements (the apex of d10, d11 and the following polypyrimidine tract) that are homologous to corresponding elements in canonical Type 1 IRESs, and non-canonical flanking domains (d8, d9 and d12). In vitro reconstitution revealed that as with canonical Type I IRESs, 48S complex formation requires eukaryotic initiation factors (eIFs) 1, 1A, 2, 3, 4A, 4B and 4G, and the poly(C) binding protein 2 (PCBP2), and starts with specific binding of eIF4G/eIF4A to d11. However, in contrast to canonical Type I IRESs, subsequent recruitment of 43S ribosomal complexes does not require direct interaction of their eIF3 constituent with the IRES-bound eIF4G. On the other hand, the CDV IRES forms a 40S/eIF3/IRES ternary complex, with multiple points of contact. These additional interactions with translational components could potentially stimulate recruitment of the 43S complex and alleviate the necessity for direct eIF4G/eIF3 interaction.     

Bypassing translation initiation

A high-resolution cryo-EM reconstruction of a ribosome-bound dicistrovirus IRES (Schüler et al., 2006) and the crystal structure of its ribosome binding domain (Pfingsten et al., 2006) provide new insights into an exceptional eukaryotic translation mechanism.     

MORPHOLOGY AND ANATOMY OF FLORAL AND EXTRAFLORAL NECTARIES IN CAMPSIS (BIGNONIACEAE)

The genus Campsis (Bignoniaceae), with one New World and one Old World species, is unusual among temperate plants in having five distinct nectary sites. Multiple nectaries occur at all four of the extrafloral sites (petiole, calyx, corolla, fruit), representing an advanced strategy for ant attraction. The morphology and anatomy of the extrafloral nectaries in both species are uniform for the petioles, calyces, and young fruits; those on the outer corolla lobes are of slightly different forms. The generalized structure consists of one layer of basal cells, and a one- to two-layered secretory cup. Because of their small size, there is no vascular tissue in them. The large, vascularized (phloem only) floral nectary is an annular structure subtending the ovary.     

Inhibition of proteolytic activity of poliovirus and rhinovirus 2A proteinases by elastase-specific inhibitors.

A polyprotein cleavage assay has been developed to assay the proteolytic activities in vitro of the 2A proteinases encoded by poliovirus and human rhinovirus 14, which are representative members of the Enterovirus and Rhinovirus genera of picornaviruses, respectively. The elastase-specific substrate-based inhibitors elastatinal and methoxysuccinyl-Ala-Ala-Pro-Val-chloromethylketone (MPCMK) inhibited both 2A proteinases in vitro. The electrophoretic mobilities of both 2A proteinases were reduced upon incubation with elastatinal, whereas the mobility of a Cys-109-->Ala poliovirus 2Apro mutant was unchanged, an observation suggesting that this inhibitor may have formed a covalent bond with the active-site Cys-109 nucleophile. Iodoacetamide, calpain inhibitor 1, and antipain inhibited poliovirus 2Apro. MPCMK caused a reduction in the yields of the enteroviruses poliovirus type 1 and coxsackievirus A21 and of human rhinovirus 2 in infected HeLa cells but did not affect the growth of encephalomyocarditis virus, a picornavirus of the Cardiovirus genus. MPCMK abrogated the shutoff of host cell protein synthesis that is induced by enterovirus and rhinovirus infection and reduced the synthesis of virus-encoded polypeptides in infected cells. These results indicate that the determinants of substrate recognition by 2A proteinases resemble those of pancreatic and leukocyte elastases. These results may be relevant to the development of broad-range chemotherapeutic agents against entero- and rhinoviruses.