Fig 1. Activity and localization of Tgl during sporulation.

A: Vegetative cells of B. subtilis grow and divide at midcell (a and a’) but at the onset of sporulation the cells switch to a polar division that forms a smaller forespore (the future spore) and a larger mother cell (b). Later stages in spore differentiation include; c, initiation of engulfment; d, engulfment completion; e, formation of phase dark spores; f, final stages in the assembly of the spore protective structure; g, free spores. The main spore structures are represented at the end of the sequence. The stages in which the mother cell-specific regulatory proteins σ^E and σ^K are active are also shown. B: Reaction cycle of Tgl. The active site (only the catalytic Cys116 residues is represented) is located within a tunnel that transverses the molecule from side to side. A Gln-containing substrate (the acceptor or Q-substrate in protein A) binds to the enzyme from one side of the tunnel and forms an acyl-enzyme intermediate with the catalytic Cys116. A Lys-containing substrate (the donor or K-substrate in protein B, but note that inter-molecular reactions are also possible) then approaches the enzyme from the opposite side of the tunnel and attacks the thiolester bond, leading to the formation of a ε-(γ-glutamyl)lysil isopeptide bond between A and B. Lastly, the cross-linked substrate is released [14]. C: The figure illustrates the role of morphogenetic proteins SafA, CotE and CotZ in recruiting the inner, outer and crust proteins during spore coat assembly; SafA and CotE interact directly with the encasement protein SpoVID. SpoVM and SpoIVA are not represented for simplicity (see text for details). C30, GerQ (σ^E-controlled, pink) and YeeK (σ^K-dependent, grey) are SafA-dependent, inner coat proteins. D: structural organization of SafA. The full-length protein, SafA^FL, carries a localization signal at the N-terminal end, formed by a peptidoglycan-binding LysM domain and region A, which binds to the encasement protein SpoVID [28, 29, 49]. The internal Met codons M161 and M164 can direct production of a short form of the protein, termed C30, through internal translation of the safA mRNA. C30 binds to SafA^FL and is recruited to the coat in this way. Concomitant with the production of C30, a third form of the protein, N21, is also formed [28].