Regulation of activity of the yeast TATA-binding protein through intra-molecular interactions.

Dimerization is proposed to be a regulatory mechanism for TATA-binding protein (TBP) activity both in vitro and in vivo. The reversible dimer-monomer transition of TBP is influenced by the buffer conditions in vitro. Using in vitro chemical cross-linking, we found yeast TBP (yTBP) to be largely monomeric in the presence of the divalent cation Mg2+, even at high salt concentrations. Apparent molecular mass of yTBP at high salt with Mg2+, run through a gel filtration column, was close to that of monomeric yTBP. Lowering the monovalent ionic concentration in the absence of Mg2+, resulted in dimerization of TBP. Effect of Mg2+ was seen at two different levels: at higher TBP concentrations, it suppressed the TBP dimerization and at lower TBP levels, it helped keep TBP monomers in active conformation (competent for binding TATA box), resulting in enhanced TBP-TATA complex formation in the presence of increasing Mg2+. At both the levels, activity of the full-length TBP in the presence of Mg2+ was like that reported for the truncated C-terminal domain of TBP from which the N-terminus is removed. Therefore for full-length TBP, intra-molecular interactions can regulate its activity via a similar mechanism.

The transcriptional activator GAL4-VP16 regulates the intra-molecular interactions of the TATA-binding protein.

Binding characteristics of yeast TATA-binding protein (yTBP) over five oligomers having different TATA variants and lacking a UASGAL, showed that TATA-binding protein (TBP)-TATA complex gets stabilized in the presence of the acidic activator GAL4-VP16. Activator also greatly suppressed the non-specific TBP-DNA complex formation. The effects were more pronounced over weaker TATA boxes. Activator also reduced the TBP dimer levels both in vitro and in vivo, suggesting the dimer may be a direct target of transcriptional activators. The transcriptional activator facilitated the dimer to monomer transition and activated monomers further to help TBP bind even the weaker TATA boxes stably. The overall stimulatory effect of the GAL4-VP16 on the TBP-TATA complex formation resembles the known effects of removal of the N-terminus of TBP on its activity, suggesting that the activator directly targets the N-terminus of TBP and facilitates its binding to the TATA box.

Cloning and sequencing of complete tau-crystallin cDNA from embryonic lens of Crocodylus palustris.

tau-Crystallin is a taxon-specific structural protein found in eye lenses. We present here the cloning and sequencing of complete tau-crystallin cDNA from the embryonic lens of Crocodylus palustris and establish it to be identical to the a-enolase gene from non-lenticular tissues. Quantitatively, the tau-crystallin was found to be the least abundant crystallin of the crocodilian embryonic lenses. Crocodile tau-crystallin cDNA was isolated by RT-PCR using primers designed from the only other reported sequence from duck and completed by 5'- and 3'-rapid amplification of cDNA ends (RACE) using crocodile gene specific primers designed in the study. The complete tau-crystallin cDNA of crocodile comprises 1305 bp long ORF and 92 and 409 bp long untranslated 5'- and 3'-ends respectively. Further, it was found to be identical to its putative counterpart enzyme a-enolase, from brain, heart and gonad, suggesting both to be the product of the same gene. The study thus provides the first report on cDNA sequence of tau-crystallin from a reptilian species and also re-confirms it to be an example of the phenomenon of gene sharing as was demonstrated earlier in the case of peking duck. Moreover, the gene lineage reconstruction analysis helps our understanding of the evolution of crocodilians and avian species.