CLUSTAL W sequence alignmentbetween M.SssI and M.HhaI. Conserved motifs [2, 3, 8] are underlined and marked by Roman numerals. The TRD of M.HhaI  is in italics. The ends of N-terminal fragments constructed in this study are marked by bent symbols and the beginnings of C-terminal fragments by arrowheads above the sequence. C-terminal fragments displaying good, medium or no complementation of the [1–304] fragment are indicated by black, grey and empty arrowheads, respectively.