Cyanobacterial Zur: Zinc‐Sensing Transcriptional Regulators

Abstract

Zur (zinc uptake regulator) proteins are the most widespread bacterial zinc‐sensing transcriptional regulators, being encoded in the majority of sequenced bacterial genomes. They work predominantly as transcriptional repressors of zinc uptake genes when zinc in a bacterium's environment is abundant. Zinc scarcity in turn leads to de‐repression of these genes, which leads to increased zinc uptake. More recently, several Zur proteins have also been shown to work as activators of transcription, but the mechanism(s) for this type of regulation are not fully understood. Several 3D structures of Zur proteins are available, including those from the actinobacterium Streptomyces coelicolor, pathogenic Mycobacterium tuberculosis and Xanthomonas campestris , and the marine cyanobacterium Synechococcus sp. WH8102. The SynZur protein from the latter photosynthetic organism is the focus of this entry. In contrast to many other bacteria which harbor at least two zinc‐sensing regulators, SynZur is the sole zinc‐sensing transcription factor in this cyanobacterium. Like many other Zur proteins, it regulates the transcription of the znuABC genes, which encode ABC‐type zinc uptake systems located at the inner membrane. Uniquely, however, SynZur also activates transcription of a bacterial metallothionein ( bmtA ). The latter small cysteine‐rich proteins have a high capacity to bind up to four zinc ions and are normally regulated by repressors of the SmtB family, in response to zinc excess. The ability to upregulate production of a zinc storage protein upon encountering ‘luxury’ zinc appears to enable Synechococcus sp. WH8102 and related oligotrophic strains to populate ecological niches that are poor in inorganic nutrients but may experience episodic rises in zinc levels.