Metallothionein Gene Regulation in Cyanobacteria

Abstract

Metal-induced synthesis of animal and yeast metallothioneins is regulated at the level of transcription via known cis-acting metal-regulatory elements. Reports describing the use of such elements to control the expression of foreign genes in transgenic animals (Brinster et al. 1982; Palmiter et al. 1982, 1983) are widely cited. Trans-acting metal-responsive activatory factors that interact with these elements have been cloned and characterized from both yeasts (Thiele, 1988; Welch et al. 1989; Dameron et al. 1991; Zhou and Thiele 1993) and animals (Radtke et al. 1993, 1995; Otsuka et al. 1994; Brugnera et al. 1994; Heuchel et al. 1994). A prokaryotic metallothionein gene sequence was first determined from SynechococcusPCC 6301 (Robinson et al. 1990). Regions containing cis-acting elements involved in the regulation of the related gene from SynechococcusPCC 7942 have been identified (Huckle et al. 1993; Morby et al. 1993; Erbe et al. 1995), and one metal-responsive trans-acting factor (a repressor) has been cloned and characterized (Huckle et al. 1993; Morby et al. 1993). The mechanisms involved in metalloregulation of this cyanobacterial metallothionein gene, which clearly differ from those in eukaryotes, are described here. The functions of cyanobacterial metallothionein are also discussed because this has obvious implications for understanding the nature of its regulation and, furthermore, it is hypothesized that SmtA may itself perform a regulatory role.