Fig. 1

Types of self-splicing protein sequences and their distribution in eukaryotes. a Inteins, Hedgehog, and Hint proteins. Inteins are mobile, self-splicing protein elements present across eukarya, bacteria, and archaea. Conserved residues coordinate self-splicing, indicated by red arrows, to ligate the N-extein (blue) and C-extein (green). Hedgehog proteins are found in higher eukaryotes only and are involved in complex developmental processes. They are composed of two domains, HhN and HhC. The HhC domain is analogous to inteins, utilizing a similar mechanism to link cholesterol to HhN (red arrows). Hedgehog-intein (Hint) domains have cleavage properties similar to either the N-terminus (HintN-like) or C-terminus (HintC-like) of an intein, and are found in both metazoans and lower eukaryotes. b A modified phylogenetic tree of eukaryotes was constructed. Scaled circles indicate intein-containing phyla. In fungi, inteins are found in nuclear DNA (nDNA; red circles), in algae in chloroplast DNA (cpDNA; green circles), and in eukaryotic viruses (vDNA; blue circles). Specific intein-containing species are mentioned in the text. Total inteins in each tree are listed