Gesture and speech combine to form a rich basis for human conversational interaction. To exploit these modalities in HCI, we need to understand the interplay between them and the way in which they support communication. We propose a framework for the gesture research done to date, and present our work on the cross-modal cues for discourse segmentation in free-form gesticulation accompanying speech in natural conversation as a new paradigm for such multimodal interaction. The basis for this integration is the psycholinguistic concept of the coequal generation of gesture and speech from the same semantic intent. We present a detailed case study of a gesture and speech elicitation experiment in which a subject describes her living space to an interlocutor. We perform F. Quek et al.two independent sets of analyses on the video and audio data: video and audio analysis to extract segmentation cues, and expert transcription of the speech and gesture data by microanalyzing the videotape using a frame-accurate videoplayer to correlate the speech with the gestural entities. We compare the results of both analyses to identify the cues accessible in the gestural and audio data that correlate well with the expert psycholinguistic analysis. We show that "handedness" and the kind of symmetry in two-handed gestures provide effective supersegmental discourse cues.

In homologous recombination, a hallmark DNA strand exchange reaction is promoted by RecA-like recombinases between a single-stranded DNA (ssDNA) and a homologous double-stranded DNA (dsDNA). Such a reaction plays critical role in the repair of double-stranded DNA breaks and restart of stalled replication forks (1-5). This recombinase superfamily (6) is composed of bacterial RecA (7), archaeal RadA or Rad51 (8), and eukaryal Rad51 (9), and meiosisspecific DMC1 (10). Large differences in their primary structures exist between bacterial RecAs and nonbacterial orthologues. Their tertiary and quaternary structures, however, are clearly conserved (11-13). Such filamentous assemblies are classic allosteric systems equipped with at least two functional sites, one located at the subunit interface for binding and hydrolyzing ATP and the other located near the filament axis for binding DNA and promoting strand exchange. In vitro studies have clearly shown that optimal reaction conditions differ for the well characterized Escherichia coli RecA (EcRecA) and members of the archaeal/eukaryal group (14 -16). EcRecA favors low salt conditions with little or no monovalent cation. Human and yeast Rad51 and DMC1, on the other hand, favor the presence of a salt, typically KCl or (NH 4 ) 2 SO 4 . Fishel and coworkers further found that human Rad51 is stimulated by K ϩ or bigger monovalent cations (17). Using MvRadA 2 as a prototype, we have structurally rationalized that monovalent cations stimulate RadA/Rad51/ DMC1 by bridging and stabilizing a critical L2 region with the ATP cofactor (18, 19). However, the reported MvRadA structures could not explain the recently discovered stimulatory role by divalent Ca 2ϩ ions on human Rad51 (20) and DMC1 (21). Though initially suspected of being relevant only to higher eukaryotes in sensing the second messenger Ca 2ϩ , similar effects have later been found for yeast DMC1 (22). To test whether a similar effect by Ca 2ϩ exists in archaeal orthologues, we carried out assays on MvRadA in the presence of Ca 2ϩ . The strand exchange activity but not the ATPase activity of MvRadA is stimulated by Ca 2ϩ , suggesting that this archaeal orthologue also possesses a structural element for sensing Ca 2ϩ . We further determined the crystal structure of MvRadA in the presence of Ca 2ϩ . A largely ordered L2 is anchored by Ca 2ϩ with the ATP cofactor in an essentially identical conformation to that previously elucidated in the K ϩ -activated forms of MvRadA. Analysis of sequence and structure suggests that archaeal/ eukaryal recombinases may share a negatively charged cavity lined by ATP, the catalytic Glu, an Asp in the ATP cap, and the C terminus of a short helix in the L2 region. We interpret these results as suggesting a unified stimulation mechanism by various cations because of the existence of such a conserved and promiscuous cation-binding pocket in the ATPase center of closely related RadA/Rad51/DMC1 recombinases. * This work was supported in part by the Natural Sciences and EngineeringResearch Counci...

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