JB Accepts, published online ahead of print on 1 September 2006

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J. Bacteriol. doi:10.1128/JB.00903-06
Copyright (c) 2006, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

The Streptomyces NrdR transcriptional regulator is a Zn-ribbon/ATP-cone protein that binds to the promoter regions of class Ia and class II ribonucleotide reductase operons

Inna Grinberg, Tanya Shteinberg, Batia Gorovitz, Yair Aharonowitz, Gerald Cohen, and Ilya Borovok^*

The George S. Wise Faculty of Life Sciences, Department of Molecular Microbiology and Biotechnology, Tel Aviv University, Ramat Aviv, 69978, Tel Aviv, Israel

^* To whom correspondence should be addressed. Email: borovok{at}post.tau.ac.il.

Ribonucleotide reductases (RNRs) catalyze the conversion of ribonucleotides to deoxyribonucleotides and are essential for de novo DNA synthesis and repair. Streptomyces spp. contain genes coding for two RNRs either of which is sufficient for vegetative growth. The class Ia RNR is encoded by the nrdAB genes, the class II RNR is encoded by nrdJ which is co-expressed with nrdR. We previously showed that the Streptomyces coelicolor nrdR gene encodes a protein, NrdR that represses transcription of both sets of RNR genes. NrdR is a member of a highly conserved family of proteins that is exclusively confined to prokaryotes. In this report we describe physical and biochemical characterization of the S. coelicolor NrdR protein and show that it is a zinc-ATP/dATP containing protein that binds to the promoter regions of both Streptomyces RNR operons. The NrdR N-terminus contains a zinc-ribbon motif that is necessary for binding to the upstream regulatory region of both RNR operons. The latter contain two 16 bp direct repeat sequences, termed NrdR-boxes that are located proximal to, or overlap with, the promoter regions. These experiments support the view that NrdR controls transcription of RNR genes by binding to the NrdR-box sequences. We also show that the central NrdR ATP-cone domain binds ATP and dATP, and that mutations that abolish ATP/dATP binding significantly reduce DNA binding, suggesting that the ATP-cone domain may allosterically regulate NrdR binding. We conclude that NrdR is a widely conserved regulator of RNR genes, binding to specific sequence elements in the promoter region thereby modulating transcription.

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