Transcription-Coupled DNA Supercoiling in Escherichia Coli: Mechanisms and Biological Functions

Transcription by RNA polymerase can induce the formation of hypernegatively supercoiled DNA both in vivo and in vitro. This phenomenon has been explained by a “twin-supercoiled-domain” model of transcription where a positively supercoiled domain is generated ahead of the RNA polymerase and a negatively supercoiled domain behind it. In E. coli cells, transcription-induced topological change of chromosomal DNA is expected to actively remodel chromosomal structure and greatly influence DNA transactions such as transcription, DNA replication, and recombination. In this study, an IPTG-inducible, two-plasmid system was established to study transcription-coupled DNA supercoiling (TCDS) in E. coli topA strains. By performing topology assays, biological studies, and RT-PCR experiments, TCDS in E. coli topA strains was found to be dependent on promoter strength. Expression of a membrane-insertion protein was not needed for strong promoters, although co-transcriptional synthesis of a polypeptide may be required. More importantly, it was demonstrated that the expression of a membrane-insertion tet gene was not sufficient for the production of hypernegatively supercoiled DNA. These phenomenon can be explained by the “twin-supercoiled-domain” model of transcription where the friction force applied to E. coli RNA polymerase plays a critical role in the generation of hypernegatively supercoiled DNA. Additionally, in order to explore whether TCDS is able to greatly influence a coupled DNA transaction, such as activating a divergently-coupled promoter, an in vivo system was set up to study TCDS and its effects on the supercoiling-sensitive leu-500 promoter. The leu-500 mutation is a single A-to-G point mutation in the -10 region of the promoter controlling the leu operon, and the AT to GC mutation is expected to increase the energy barrier for the formation of a functional transcription open complex. Using luciferase assays and RT-PCR experiments, it was demonstrated that transient TCDS, “confined” within promoter regions, is responsible for activation of the coupled transcription initiation of the leu-500 promoter. Taken together, these results demonstrate that transcription is a major chromosomal remodeling force in E. coli cells.

Infants' selective attention to faces and prosody of speech: The roles of intersensory redundancy and exploratory time

One of the overarching questions in the field of infant perceptual and cognitive development concerns how selective attention is organized during early development to facilitate learning. The following study examined how infants' selective attention to properties of social events (i.e., prosody of speech and facial identity) changes in real time as a function of intersensory redundancy (redundant audiovisual, nonredundant unimodal visual) and exploratory time. Intersensory redundancy refers to the spatially coordinated and temporally synchronous occurrence of information across multiple senses. Real time macro- and micro-structural change in infants' scanning patterns of dynamic faces was also examined. ^ According to the Intersensory Redundancy Hypothesis, information presented redundantly and in temporal synchrony across two or more senses recruits infants' selective attention and facilitates perceptual learning of highly salient amodal properties (properties that can be perceived across several sensory modalities such as the prosody of speech) at the expense of less salient modality specific properties. Conversely, information presented to only one sense facilitates infants' learning of modality specific properties (properties that are specific to a particular sensory modality such as facial features) at the expense of amodal properties (Bahrick & Lickliter, 2000, 2002). ^ Infants' selective attention and discrimination of prosody of speech and facial configuration was assessed in a modified visual paired comparison paradigm. In redundant audiovisual stimulation, it was predicted infants would show discrimination of prosody of speech in the early phases of exploration and facial configuration in the later phases of exploration. Conversely, in nonredundant unimodal visual stimulation, it was predicted infants would show discrimination of facial identity in the early phases of exploration and prosody of speech in the later phases of exploration. Results provided support for the first prediction and indicated that following redundant audiovisual exposure, infants showed discrimination of prosody of speech earlier in processing time than discrimination of facial identity. Data from the nonredundant unimodal visual condition provided partial support for the second prediction and indicated that infants showed discrimination of facial identity, but not prosody of speech. The dissertation study contributes to the understanding of the nature of infants' selective attention and processing of social events across exploratory time.^

Metacognitive Functions, Interest, and Student Engagement in the Writing Process: A Review of the Literature

The purpose of this paper is to draw on research that discusses the relationship between interest and metacognitive functions and its effect on engaging students in the writing process. Results indicate students who are interested in their writing activities engage in metacognitive strategies, remain focused, and complete their tasks.