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1. Introduction
A protein isolated in 1992 from Escherichia coli cells can bind DNA in vitro and was proposed to protect DNA from oxidative damage [1]. Its name DNA-binding protein from starved cell (Dps) has been widely accepted. However, some proteins belonging to the same family, which was isolated earlier, do not bind DNA, that is, the 4D antigen of Treponema pallidum isolated in 1987 [2], which was also known as TpF1 [3] and C1-5 [4]. They were proposed to play an important structural role in the outer membrane of the bacteria. Immunoelectron microscopy confirmed its surface localization [2], but it precipitated differently from other membrane proteins in ultracentrifugation [5].
The DNA-binding activity of the E. coli Dps protein is due to a nonspecific interaction between the positively charged N-terminus and the negatively charged phosphate group of DNA backbone (Figure 1(a)). The protein isolated from Agrobacterium tumefaciens lacks such an N-terminal and hence does not bind DNA [6]. The protein from Mycobacterium smegmatis can bind DNA, but duet to a positively charged C-terminus [7]. It lost the capability as soon as the C-terminus was truncated.
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All of the known Dps-like proteins are composed of small subunits of about 20 kD (Table 1). They usually form an aggregate of 12 subunits with a central hollow cavity. Ferrous ions bind to the dinuclear ferroxidase sites at dimer interfaces [8–11]. Their proposed functions include oxidative detoxification, resistance to toxic electrophile and acid stress and so forth [12–14]. Many Dps proteins contain a large amount of iron. The iron sequestration of the E. coli Dps has also been detected [15]. A Dps-like protein, MrgA, was identified in Bacillus subtilis in a search for metallo-regulated genes [16]. MrgA is also inducible by starvation and protects the cell against oxidative stress [17]. Another Dps-like protein was isolated from Listeria innocua, and named ferritin-like protein (Flp), because it can sequester a large amount of iron in its central cavity [18]. The expression of flp genes in L. innocua and Listeria monocytogenes is not only inducible by both stationary phase and low iron availability [19] but also by cold shock [20]. Another Dps-like protein, the Neutrophil-activating protein (Nap), was isolated from Helicobacter pylori [21]. Nap was thought...