. Le bulletin biologique. Biologie ; zoologie ; biologie ; biologie marine. SZENT-GYORGYI SYMPOSIUM 211 tein, appelé facteur de transcription IIIA (TFIIIA). Nous avons purifié cette protéine, et ont montré qu'il a un re- marquables répétition des actions. Chaque unité structurale ou "mini-domaine" se compose d'une petite boucle d'environ 30 acides aminés repliés autour d'un ion de zinc. Les unités ont une proportion relativement élevée d'acides aminés conservés, y compris quatre résidus invariantes à l'ion zinc ligaturées, qui peuvent former un cadre structurel pour les domaines chimiquement distinctes. Chaque unité peut-il y

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. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. SZENT-GYORGYI SYMPOSIUM ABSTRACTS 211 tein, called Transcription Factor IIIA (TFIIIA). We have purified this protein, and have shown that it has a re- markable repeating structure. Each structural unit or "mini-domain" consists of a small loop of about 30 amino acids folded around a zinc ion. The units have a relatively high proportion of conserved amino acids, including four invariant residues ligated to the zinc ion, which can form a common structural framework for chemically distinct domains. Each unit can therefore rec- ognize a different short stretch of DNA. We have called these domains "Zn fingers, " and have further shown that each finger binds to, and thereby recognizes the sequence on, about half a turn of the DNA double helix. As we predicted, these nucleic acid binding fingers have turned up in many other regulatory proteins. It thus appears that TFIIIA is the type of a novel class of proteins whose modular design offers a large number of combina- torial possibilities for the specific recognition of many different nucleic acid sequences. Two subclasses appear to have emerged, according to whether the zinc ion is ligated to a pair of cysteines and a pair of histidines or else to two pairs of cysteines. It remains to be seen whether this type of fold occurs in other proteins not involved in nucleic acid recognition. Aaron Klug was born in 1926, educated at the Universities of Witwatersrand and Cape Town, and was an 1851 Exhibition Overseas Scholar in the Caven- dish Laboratory, Cambridge (1949-1952). He was research Fellow at Birkbeck College (1953-1961) where he worked with Rosalind Franklin (until her death in 1958) and thereaf- ter led a team working on virus structure. Since 1962 he has been a member of staff at the Medical Research Council Lab- oratory of Molecular Biology, Cambridge. (Joint Head of the ^ Division of Structural Studies, 1978-1986. Director since 1

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