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Abstract

The best-characterized DNA-binding protein structure is the evolutionarily-conserved helix-turn-helix motif. Recently a second motif for DNA-binding proteins, the 'zinc finger', emerged from sequence analysis of TFIIIA, a factor involved in the control of transcription of the Xenopus 5S RNA gene. The finger structure is based on pairs of Cys and His residues which are arranged around a tetrahedrally-coordinated zinc ion. This centre allows the folding of tandemly repeated 'finger loops' which are thought to specify the contact with target DNA. Zinc fingers have been observed in the DNA-binding protein domains of transcriptional activators in yeast and man (R. Tijan, personal communication) and in several regulatory proteins of Drosophila including proteins encoded by members of the gap class of segmentation genes. One of these, Krüppel (Kr), acts at the first level of the segmentation gene hierarchy, and its protein product may bind to DNA. In addition, Kr is required for the development of the malpighian tubules, a posterior internal tissue that forms during later stages of embryogenesis. Here we show that a mutation which results in a conservative amino-acid exchange eliminates Kr+ function. The change occurs in a key position within the putative core structure of a finger, and supports the role of Cys in metal binding as proposed by Klug and coworkers.