A complete amino acid sequence for the rubredoxin from the photosynthetic bacterium Chlorobium thiosulphatophilum is proposed. The sequence, a single polypeptide chain of 53 amino acids, was deduced from the sequences of peptides obtained by chymotryptic, tryptic, thermolytic or mild acid digestion.The rubredoxin shows a high degree of sequence homology with rubredoxins from non-photosynthetic bacteria, and the evolutionary implications of this are considered. [6]. Clostridium thermoaceticum has been shown (uniquely) to contain two distinct rubredoxins [7]. The molecular masses of all these proteins are about 6 kDa and the redox potentials about -50 mV at pH 7.0. Amino acid sequences have been reported for the rubredoxins from P . aerogenes [8], D . gigas [9], M . elsdenii [lo], D . vulgaris [ l l ] and C . pasteurianum [12].All these rubredoxins contain a single iron atom, but no inorganic sulphur. X-ray structure analysis showed that the iron atom is tetrahedrally chelated to the sulphur atoms of the four cysteine residues in the amino acid chain [13 -171.Only two rubredoxins have been isolated from obligate aerobes; Pseudomonas oleovorans [18] and Acinetobacter calcoaceticus [19]. The Ps. oleovorans rubredoxin is unusual in that it is about twice the size of the C. pasteurianum rubredoxin; sequence data suggest that this was caused by an ancestral gene duplication [20]. The Ps. oZeovorans rubredoxin has been shown to function as an electron carrier in an enzyme system that hydroxylates alkanes and fatty acids [18,21]. The alkane-oxidizing system of Ac. calcoaceticus may function in a similar manner [22]. In contrast the functional role of rubredoxin in anaerobic bacteria is largely unknown, although there is some evidence to suggest that it is the preferred electron donor for carbon monoxide dehydrogenase in Cl. thermoaceticum [23, 241. Rubredoxin has not been detected in the purple sulphur or non-sulphur photosynthetic bacteria, but appears to be generally present in green sulphur bacteria isolated rubredoxin from the green sulphur bacterium Chlorobium thiosulphatophilum, the complete amino acid sequence of which is reported in the present paper.
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