No abstract

The long-term viability of virtual communities depends critically on contribution behavior by their members. We deepen and extend prior research by conceptualizing contributions to virtual communities in terms of small friendship group-referent intentional actions. Specifically, we investigate cognitive, emotional, and social determinants of shared we-intentions and their consequences for member contribution behavior to the small friendship group to which they belong within a larger community. Using multiple measurement sources and a longitudinal quasi-experimental design, we show that group norms and social identity, as well as attitudes and anticipated emotions, contribute to the development of behavioral desires, which in turn influence weintentions. In addition, subjective norms are less effective than either group norms or social identity in encouraging contribution behavior. Finally, members' experience levels positively moderate the relationship between we-intentions and contribution behaviors, and differences between collectivistic versus individualistic orientations moderate the effects of social identity and anticipated emotions on the desire to contribute to one's friendship group in the virtual community. Tests for methods biases were conducted, as well as rival hypotheses. These findings have significant research and managerial implications.

RNase P, which catalyzes the magnesium-dependent 5-end maturation of tRNAs in all three domains of life, is composed of one essential RNA and a varying number of protein subunits depending on the source: at least one in bacteria, four in archaea, and nine in eukarya. To address why multiple protein subunits are needed for archaeal͞eukaryal RNase P catalysis, in contrast to their bacterial relative, in vitro reconstitution of these holoenzymes is a prerequisite. Using recombinant subunits, we have reconstituted in vitro the RNase P holoenzyme from the thermophilic archaeon Pyroccocus furiosus (Pfu) and furthered our understanding regarding its functional organization and assembly pathway(s). Whereas Pfu RNase P RNA (RPR) alone is capable of multiple turnover, addition of all four RNase P protein (Rpp) subunits to Pfu RPR results in a 25-fold increase in its k cat and a 170-fold decrease in Km. In fact, even in the presence of only one of two specific pairs of Rpps, the RPR displays activity at lower substrate and magnesium concentrations. Moreover, a pared-down, mini-Pfu RNase P was identified with an RPR deletion mutant. Results from our kinetic and footprinting studies on Pfu RNase P, together with insights from recent structures of bacterial RPRs, provide a framework for appreciating the role of multiple Rpps in archaeal RNase P.archaeal RNase P ͉ in vitro reconstitution ͉ precursor tRNA processing R ibonuclease P (RNase P) is an ancient and essential endoribonuclease that catalyzes the 5Ј-end maturation of tRNAs in all three domains of life (1-7). Whereas RNase P in all living organisms contains one essential RNA subunit, the number of protein cofactors͞subunits varies: at least one in bacterial (8), four in archaeal (9), and nine in eukaryal (nuclear) RNase P (10, 11). The basis for this variation, which has implications for macromolecular evolution, is unclear.The bacterial RNase P RNA (RPR) alone is catalytically active under in vitro conditions of high ionic strength in the presence of a divalent ion such as Mg 2ϩ (1); however, the protein cofactor is essential for RNase P function in vivo because of its pleiotropic effects on RNA structure, substrate recognition, affinity for Mg 2ϩ , and precursor tRNA (ptRNA) cleavage (12-19). Although phylogenetic sequence analysis revealed that archaeal and eukaryal RPRs likely retain the same catalytic core as the bacterial ribozyme, many archaeal and all eukaryal RPRs seem to be incapable of supporting catalysis in the absence of their protein cofactors (6,7,20,21). Genetic and biochemical studies established the association of yeast͞human nuclear RNase P activity with at least nine protein subunits (seven of which are homologs) (10, 11). RNase P activity could be immunoprecipitated from a partially purified Methanothermobacter thermoautotrophicus (Mth) RNase P preparation by using polyclonal antisera generated against four Mth polypeptides that exhibit sequence homology to four of the seven conserved yeast͞human RNase P proteins (Rpps; ref. 9). None of the archaeal...

Few and limited amino acid sequence homologies have been found among eight bacterial aminoacyl transfer RNA (tRNA) synthetases whose primary structures are known. The entire 939-amino acid primary structure of Escherichia coli isoleucyl-tRNA synthetase is now reported. In a sequence of 11 consecutive amino acids matching a sequence in E. coli methionyl-tRNA synthetase, there are ten identical residues and one conservative change. This is the strongest homology recorded between any two aminoacyl tRNA synthetases. This part of the methionine enzyme's three-dimensional structure has been determined, and it occurs in a mononucleotide binding fold; a close three-dimensional structural homology of this part of the enzyme with Bacillus stearothermophilus tyrosyl-tRNA synthetase has also been reported. The three synthetases probably fold identically in this region.