Prokaryotic metallothionein

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Abstract
A unique metallothionein (MT) has been isolated from a Synechococcus sp. cyanobacterium by gel permeation and reverse phase high performance liquid chromatography. Although physiologically similar to well characterized eukaryotic MTs, amino acid analysis and amino terminal sequence determination show substantial differences. The cysteine content is 18% and the molecule contains 3 histidines, 2 tyrosines and 7 long chain aliphatics compared with approximately 32% cysteine content and none or very few of the latter three residues in eukaryotic compositions. On determination of 43% of the prokaryotic primary structure only the presence of Cys‐X‐Cys sequences indicated a structure similar to eukaryotic MTs and on alignment for maximum homology only 3 out of the 24 residues were identical when compared with any of 3 eukaryotic sequences. Allowing for conservative homologous replacements increased the homology to only 4 amino acids. Lack of association of serine and Iysine residues with cysteines in the sequenced portion of the bacterial MT, together with the previous distinctions, suggests major tertiary structure differences with eukaryotic MTs.