Fig. 1. Alignment of the Dd-STATa, b and c sequences after removal of their simple sequence component. The Accession Number of the complete predicted sequence of Dd-STATb is AJ581661 but here a truncated form of the sequence is presented. The N-terminal halves of Dd-STATa, b and c contain tracts of glutamine, asparagines and threonine. These are encoded by CAA repeats, a feature common to many Dictyostelium genes. In this alignment, Q, N and T tracts equal to or longer than three residues were omitted, to give Dd-STATa' (633 of 707 residues), Dd-STATb' (978 of 1147 residues)and Dd-STATc' (819 of 929 residues). The three STATs display only scattered regions of short homology in their N-terminal-proximal regions. No functions have thus far been mapped to the N-terminal-proximal regions of Dd-STATa or Dd-STATc and a BLAST search using the N-terminal-proximal region of Dd-STATb also yielded no hits (the search was run at NCBI with amino acids 1 to 505 and using blastP with an `E' value of 10). The predicted approximate positions of the DNA binding domains (closely spaced broken line), the SH2 domains (widely spaced broken line) and the site of the insertion in the Dd-STATb sequence (broad unbroken line) are indicated by double-headed arrows. The positions of the arginine to leucine substitution is indicated by a triangle, and the predicted site of tyrosine phosphorylation is indicated by an asterisk. (B) Alignment between the SH2 domains of Dd-STATs a to c, human STATs 1 to 6 and Src. The alignment was generated using ClustalW and then modified by hand to align the known secondary structure elements from STAT1 and the Src SH2 domain. This alignment was used in modelling the Dd-STATb SH2 domain (STATc). The dominant inserts are highlighted in yellow; red indicates identical residues; grey indicates similar residues. The inset shows a phylogenetic tree generated using the Nearest Neighbour joining method; blue boxes indicate proteins where a crystal structure is known.