No abstract

The promoters of a variety of plant genes are characterized by the presence of a G‐box (CCACGTGG) or closely related DNA motifs. These genes often exhibit quite diverse expression characteristics and in many cases the G‐box sequence has been demonstrated to be essential for expression. The G‐box of the Arabidopsis rbcS‐1A gene is bound by a protein, GBF, identified in plant nuclear extracts. Here we report the isolation of three Arabidopsis thaliana cDNA clones encoding GBF proteins referred to as GBF1, GBF2 and GBF3. GBF1 and GBF2 mRNA is present in light and dark grown leaves as well as in roots. In contrast, GBF3 mRNA is found mainly in dark grown leaves and in roots. The deduced amino acid sequences of the three cDNAs indicate that each encodes a basic/leucine zipper protein. In addition, all three proteins are characterized by an N‐terminal proline‐rich domain. Homodimers of the three proteins specifically recognize the G‐box motif, with GBF1 and GBF3 binding symmetrically to this palindromic sequence. In contrast, GBF2 binds to the symmetrical G‐box sequence in such a way that the juxtaposition of the protein and the DNA element is clearly asymmetric and hence distinct from that observed for the other two proteins. The fact that GBF1, GBF2 and GBF3 possess both distinct DNA binding properties and expression characteristics prompt us to entertain the notion that these proteins may individually mediate distinct subclasses of expression properties assigned to the G‐box. Furthermore, we demonstrate that GBF1, GBF2 and GBF3 heterodimerize and these heterodimers also interact with the G‐box, suggesting a potential mechanism for generating additional diversity from these GBF proteins.

A fourth member of the Aralopm G-boxb ing factor (GBF) family of bZIP proteins, GBF4, has been solated and characterized. In a manner remst of the Fos-related oncoproteins of mam n systems, GBF4 cannot bind to DNA as a ho er, although it coins a basic region cpable of spc fi g the G-box and G-boxlike elements. However, GBF4 can interact with GBF2 and GBF3 to bind DNA as hetrdmers. Mutagenesis ofthe leuine zipper of GBF4 indiates that the mutation of a single amino add coanfs upon the protein the ability to r ize the G-box as a hodimer, apparently by altering the charge distribution within the leucne zipper.The mechanisms of regulated eukaryotic gene expression are topics of intense examination. These regulatory pathways require the coordination of highly specific DNA-protein and protein-protein interactions, many of which are not fully understood. One goal is to understand the individual contribution of seemingly similar DNA elements to regulated gene expression when these conserved elements are present in promoters responding to diverse regulatory controls. In plant systems, one example of this complexity is illustrated by the hexameric G-box (5'-CACGTG-3') and those DNA elements that contain small variations on this palindromic sequence, which we refer to collectively as G-box-like elements. In some cases these elements have been identified as targets for nuclear DNA-binding factors, examples of which include the light-regulated genes of ribulose-1,5-bisphosphate carboxylase/oxygenase (1), chlorophyll a/b binding proteins (2), and chalcone synthase (3), genes regulated by abscisic acid such as the Em gene of wheat (4) and the rabl6A gene of rice (5), as well as the stress-induced Adh gene of maize and Arabidopsis (6, 7). The identification of these nuclear factors has prompted the isolation and characterization of 10 DNAbinding proteins specific for the G-box and G-box-like elements from four different plant species (4,(8)(9)(10)(11).To understand how a seemingly ubiquitous element can be involved in such diverse regulatory pathways, it is essential to understand the individual contributions made by each of the relevant DNA-binding proteins. To date, all of the plant DNA-binding proteins that recognize the G-box belong to the bZIP family of transcription factors (12, 13). These proteins are characterized by the presence of the basic region, a subdomain of -20 residues rich in basic amino acids that mediates DNA binding. Immediately adjacent to the basic region is the leucine zipper, a dimerization motif defined by a 4-3 repeat of typically four or five leucine residues interspersed with other hydrophobic amino acids, which align in parallel to form a coiled.coil, with the leucine and additional hydrophobic residues forming a hydrophobic interface (14)(15)(16)(17)(18).A well-documented example of the intricate interactions between members of a family of related bZIP proteins is illustrated with the products ofthefos andjun oncogenes (12,13,19). There are three Jun-related proteins, each of which is capa...

Although dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) and magnetic resonance spectroscopy (MRS) have great potential to provide routine assessment of cancer treatment response, their widespread application has been hampered by a lack of standards for use. Thus, the National Cancer Institute convened a workshop to assess developments and applications of these methods, develop standards for methodology, and engage relevant partners (drug and device industries, researchers, clinicians, and government) to encourage sharing of data and methodologies. Consensus recommendations were reached for DCE-MRI methodologies and the focus for initial multicenter trials of MRS. In this meeting report, we outline the presentations, the topics discussed, the ongoing challenges identified, and the recommendations made by workshop participants for the use of DCE-MRI and 1 H MRS in the clinical assessment of antitumor therapies. Workshop GoalsIn the quest to tailor and fine-tune cancer therapies, clinicians may exploit the capacity of magnetic resonance to produce rapid, accurate, in vivo assessment. Two methods in particular, dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) and magnetic resonance spectroscopy (MRS), have outstanding potential. In recent years, they have been advancing the field of assessment and treatment significantly, despite the fact that issues such as standardization and reliability have limited their broad use.MRI has served in clinical cancer detection, diagnosis, and intervention and has been employed in the development of drug therapies. There is a need to move beyond the many isolated successes of magnetic resonance applications to widespread routine use of these tools to assess response to cancer treatment. This will require overcoming hurdles that are logistic rather than scientific. It will require the establishment of standards of use, leading to comparable data and reliability.On November [22][23] 2004 in Bethesda, Maryland, the National Cancer Institute (NCI) convened a workshop on the use of MRI, MRS, and related technologies. Presentations were combined with strategy sessions that focused on ways to assess developments and applications of MRI and MRS, to develop standards for methods of use, and to engage relevant partners (drug and device industries, researchers, clinicians, and government) to encourage the sharing of data and methodologies. The meeting participants also identified ongoing challenges. The main points of discussion from the meeting are given below. Perspectives from the FieldThe oncologist must choose from an array of new drugs, many of which share targets in cancer treatment. MRI and MRS can help identify drug effects within a tumor, revealing which drugs work best, alone or in combination. To the oncologist, some drug effects may be more important than tumor shrinkage, and magnetic resonance can reveal these effects. MRI and MRS can be important tools in new drug development, especially to show biological activity and evaluate pharmacokin...

The purpose of writing this review is to showcase the Molecular Imaging and Contrast Agent Database (MICAD; www.micad.nlm.nih.gov) to students, researchers and clinical investigators interested in the different aspects of molecular imaging. This database provides freely accessible, current, online scientific information regarding molecular imaging (MI) probes and contrast agents (CA) used for positron emission tomography, single-photon emission computed tomography, magnetic resonance imaging, x-ray/computed tomography, optical imaging and ultrasound imaging. Detailed information on >1000 agents in MICAD is provided in a chapter format and can be accessed through PubMed. Lists containing >4250 unique MI probes and CAs published in peer-reviewed journals and agents approved by the United States Food and Drug Administration (FDA) as well as a CSV file summarizing all chapters in the database can be downloaded from the MICAD homepage. Users can search for agents in MICAD on the basis of imaging modality, source of signal/contrast, agent or target category, preclinical or clinical studies, and text words. Chapters in MICAD describe the chemical characteristics (structures linked to PubChem), the in vitro and in vivo activities and other relevant information regarding an imaging agent. All references in the chapters have links to PubMed. A Supplemental Information Section in each chapter is available to share unpublished information regarding an agent. A Guest Author Program is available to facilitate rapid expansion of the database. Members of the imaging community registered with MICAD periodically receive an e-mail announcement (eAnnouncement) that lists new chapters uploaded to the database. Users of MICAD are encouraged to provide feedback, comments or suggestions for further improvement of the database by writing to the editors at: micad@nlm.nih.gov