Alignment of the putative E. histolytica IscU and IscS with homologs from C. jejuni , A. vinelandii , R. prowazekii , S. cerevisiae , and H. sapiens . A. Alignment of the E. histolytica long-form IscU with similar isoforms from C. jejuni and A. vinelandii. In addition, short-form IscU homologs from R. prowazekii, S. cerevisiae, and H. sapiens are aligned concatenated with their Nfu1 homologs (arrow indicates start of Nfu1 homologs) which resemble the C-terminal extension found on the long-form IscU. The conserved cysteine residues which provide a scaffold for the IscS-directed sequential assembly of labile FeS-clusters  are boxed. The cysteine residue that forms a disulfide bridge with a conserved cysteine residue on IscS (see B) is indicated by a closed square (■). The yeast Nfu1 mitochondrial transit peptide has been deleted. B. Alignment of the E. histolytica IscS with homologs from the above mentioned organisms. Important residues for function are as described by Tachezy, Sánchez and Müller ; the conserved lysine involved in co-factor binding (pyridoxal-5'-phosphate, PLP) is indicated by a closed circle (●), other residues involved in PLP interaction are indicated by open circles (○), the cysteine residue that forms a disulfide bridge with a cysteine residue on IscU (see A) is indicated by a closed box (■), residues involved in substrate binding (L-cysteine) are indicated by open squares (□), the conserved histidine involved in substrate deprotonation is indicated by an arrow. Typical eukaryotic/eubacterial conserved cysteine and C-terminal residues are boxed. Note that organisms that contain a long-form IscU (see Fig. 2B and 3A) do not have these conserved residues suggesting that the C-terminal IscU extension might take over the role of these residues. Part of the mitochondrial transit peptides from the yeast and human IscS homologs have been omitted (~) for reasons of clarity. Amino acids were shaded according to similarity/identity scores: dark grey indicates fully conserved residues while light grey indicates similar residues according to the PAM250 matrix .