The Einstein Tower was the product of the complementary investigations of expressionism, reinforced concrete construction, and relativity undertaken by its architect, Erich Mendelsohn, between 1912 and 1920. The war-ravaged German economy of 1921, which impeded its construction, and the scientific agenda of its patron, Erwin Finlay Freundlich, which determined the character of its interior spaces, also helped shape its final appearance. Designed to serve scientific inquiry, it occupies a distinctive intellectual, as well as stylistic, position within the history of German expressionism. In this building Mendelsohn established the design approach that would characterize the rest of his German career, fusing attention to program with bold images of the thrilling instability of modern life. As its reception demonstrates, the functional aspects of the tower have been overshadowed by the degree to which its form has mistakenly been identified with a contemporary enthusiasm for mysticism, which in fact played no role in its design.

Recent studies suggest that a novel second messenger, cyclic d-GMP (c-diGMP), is extensively used by bacteria to control multicellular behaviour. This cyclic dinucleotide is synthesised by the diguanylate cyclase (DGC) domain in a reaction that converts two GTP into one c-diGMP and two pyrophosphates. The DGC domain contains a highly conserved GG(D/E)EF sequence motif, and occurs in various combinations with sensory / regulatory domains in bacteria. We have identified the response regulator, PleD, from Caulobacter crescentus as a diguanylate cyclase [1] and have solved its crystal structure in complex with c-diGMP to 2.7 Å [2]. PleD consists of a receiver domain D1 with a phosphorylation site, a receiver-like domain D2, and an effector domain DGC. In the structure, PleD forms a homodimer mediated by D1-D2 interactions. The DGC domain has a similar fold as the catalytic domain of adenylate cyclase but has an active site that reveals different nucleotide binding. The guanine base of c-diGMP is hydrogen bonded to Asn335 and Asp344, while the ribosyl and-phosphate groups extend over the 2-3 hairpin that carries the sequence motif. Interestingly, the c-diGMP molecule crosslinks two symmetrically arranged DGC domains from adjacent dimers. We propose that activation of PleD through phosphorylation leads to dimerisation, which allows the two DGC domains of a dimer to align symmetrically for c-diGMP synthesis. Two intercalated c-diGMP molecules are bound to the domain interface between D2 and DGC. This allosteric binding site explains the observed non-competitive product inhibition. We propose that PleD inhibition is effected by DGC domain immmobilisation to the D1-D2 stem.

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