The AT-rich interaction domain (ARID) is a DNAbinding module found in many eukaryotic transcription factors. Using NMR spectroscopy, we have determined the ®rst ever three-dimensional structure of an ARID±DNA complex (mol. wt 25.7 kDa) formed by Dead ringer from Drosophila melanogaster. ARIDs recognize DNA through a novel mechanism involving major groove immobilization of a large loop that connects the helices of a non-canonical helix±turn±helix motif, and through a concomitant structural rearrangement that produces stabilizing contacts from a b-hairpin. Dead ringer's preference for AT-rich DNA originates from three positions within the ARID fold that form energetically signi®cant contacts to an adenine±thymine base step. Amino acids that dictate binding speci®city are not highly conserved, suggesting that ARIDs will bind to a range of nucleotide sequences. Extended ARIDs, found in several sequence-speci®c transcription factors, are distinguished by the presence of a C-terminal helix that may increase their intrinsic af®nity for DNA. The prevalence of serine amino acids at all speci®city determining positions suggests that ARIDs within SWI/SNF-related complexes will interact with DNA non-sequence speci®cally.

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Xis protein enhances excision in a manner similar to the excisionase protein of bacteriophage , serving an architectural role in the stabilization of protein-nucleic acid structures required for strand synapsis. However, our finding that excision in E. coli is significantly enhanced by the host factor HU, but does not depend on the integration host factor or the factor for inversion stimulation, defines clear mechanistic differences between Tn916 and bacteriophage recombination.

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We have isolated a cytokinin up-regulated cDNA clone, H13, from an early stage of cultured tobacco mesophyll protoplasts by a differential display method. The expression of this gene was specifically induced by natural and synthetic cytokinins including N-(2-chloro-4-pyridyl)-N'-phenylurea (4PU30), a diphenylurea-type cytokinin, although the simultaneous presence of auxin was also required. It seems that the preceding treatment of the tobacco mesophyll protoplasts by auxin is necessary for the gene to respond to cytokinin. The addition of a cytokinin antagonist, compound 182, which suppressed the induction of cell division in tobacco mesophyll protoplasts, completely abolished the expression of this gene. Though the predicted gene product of H13 did not suggest us any sequences of defined functions, two domains of the predicted sequence had significant homology to several reported sequences in the data base. The gene product of H13 is proposed to have a role in regenerating cell wall in cultured protoplasts, since a cDNA clone E6, from cotton fiber cells, which has the most closely related structure to H13, has been isolated from cells which showed active cellulose synthesis. This supposition is supported by the evidence that in the absence of cytokinin, cell wall regeneration was significantly suppressed, resulting in failure of the induction of cell division. Thus, the gene product of H13 is supposed to have a role in regenerating cell walls and facilitating the progression of the cell cycle, resulting in the sustained cell division of tobacco mesophyll protoplasts.