The tobacco bZIP transcription factor BZI-1 binds to G-box elements in the promoters of phenylpropanoid pathway genes in vitro, but it is not involved in their regulation in vivo

Screening of a tobacco (Nicotiana tabacum) cDNA library resulted in the isolation of a clone encoding the bZIP transcription factor BZI-1. With respect to amino acid sequence, conservation of protein domains, genomic exon-intron structure and expression pattern, BZI-1 is closely related to CPRF2, OHP1/2, BLZ1 and REB, a group of bZIP proteins which have been described in a number of dicot and monocot species. BZI-1 exhibits the characteristics of a transcription factor. It binds to G-box and C-box cis-elements in vitro, it is localised in the nucleus, and the N-terminal region of BZI-1 functions as an activation domain in both yeast and plant cells. Since BZI-1-related transcription factors have been isolated from dicots by in vitro binding to G-box elements in the chalcone synthase ( CHS) promoter, it has been suggested that phenylpropanoid pathway genes, such as CHS and PAL (phenylalanine ammonia-lyase), are targets of these proteins in vivo. However, after infection with Pseudomonas syringae or Tobacco Mosaic Virus, no changes in pathogen-induced PAL expression were observed in transgenic plants expressing increased levels of BZI-1 or a dominant negative form of the protein, BZI-1-DeltaN. In contrast to the tissue-specific expression of CHS and PAL, BZI-1 was found to be ubiquitously expressed in tobacco plants. Furthermore, no changes in the tissue-specific expression of PAL or CHS were observed in plants that were transgenic for BZI-1-DeltaN. Expression of a VP16-BZI-1 fusion protein would be expected to result in constitutive activation of the BZI-1 target genes. However, tetracycline-dependent expression of a VP16-BZI-1 protein in tobacco plants did not result in activation of CHS or PAL. On the basis of these data, we conclude that the phenylpropanoid pathway genes analysed are not targets of BZI-1 in vivo. Thus, the pattern of in vitro DNA binding of transcription factors need not always reflect their in vivo function.

BZI-1 specifically heterodimerises with the tobacco bZIP transcription factors BZI-2, BZI-3/TBZF and BZI-4, and is functionally involved in flower development

The tobacco (Nicotiana tabacum) protein BZI-1 is closely related to the plant bZIP transcription factors CPRF2, G/HBF-1 and OHP1. Using the C-terminal part of BZI-1, which includes the bZIP domain, as a bait in a yeast two-hybrid screen, three BZI-1 interacting bZIP transcription factors, referred to as BZI-2, BZI-3/TBZF and BZI-4, were isolated. The observed interactions are due to the leucine zipper dimerisation domain and have been found to be specific, in so far as other bZIP transcription factors do not interact with BZI-1. The formation of heterodimers is favoured to homodimerisation. Furthermore, physical protein-protein interaction was confirmed by in vitro binding studies. Expression analysis reveals that BZI-2 mRNA is predominantly located in the stems and parts of the flower. BZI-3/TBZF expression has been observed predominantly in flowers and, to a lesser extent, in the vegetative parts of the plant. In particular, BZI-4 is transcribed specifically in the stamen, the petals and the pistils of the tobacco flower. Since BZI-1 is expressed ubiquitously in tobacco plants, co-localisation with BZI-2, BZI-3/TBZF or BZI-4 might influence BZI-1 heterodimerisation and consequently target gene selection. Analysis of transgenic plants displaying changes in BZI-1 protein level revealed that BZI-1 regulates BZI-4 expression. Moreover, a reduction in functional BZI-1 protein resulted in flowers having a reduced size; in particular, the stamen and the petals are affected. Consequently, BZI-1 homo- or heterodimers are involved in the control of the size of the organs of flowers. Based on these data, we discuss a model postulating BZI transcription factor heterodimerisation as a mechanism determining target gene selection and regulating processes involved in plant development.

Heparin cofactor II, antithrombin-beta and their complexes with thrombin in human tissues

In the presence of glycosaminoglycans, thrombin is rapidly inactivated by two natural inhibitors secreted from liver: antithrombin (AT) is presumed to be the principal thrombin inhibitor in circulating blood, while for heparin cofactor II (HCII), a role outside circulation has been proposed. In this study, we show that HCII and AT differ with respect to their association with human tissues. Aside from brain, each of these inhibitors was found in sodium dodecyl sulphate (SDS) soluble extracts of various human organs, with a preponderance of HCII in placenta. AT levels, however, predominated in liver. Compared to plasma, the beta-variant of AT was found to be strongly enriched in human organs, while tissue-resident HCII did not differ in its electrophoretic mobility from the circulating form. In placenta, comparable amounts of HCII/thrombin and AT/thrombin complexes were detected, indicating that HCII may exert a thrombin regulating role in that organ under conditions of tissue or blood vessel damage. Transcripts coding for HCII and AT were detected in all tissues examined. The low levels of their mRNAs suggest that most of the tissue-associated thrombin inhibitor molecules originate from circulation and are retained in organs, possibly by specific receptors. The differential presence of HCII and AT in organs is in accordance with individual physiological roles of these inhibitors.