Structure of the LacZ cassettes in the GoldenBraid system.

GB plasmid set comprises four destination plasmids (pDGBs), two of them act as destination plasmids for level α assembly and the remaining two function as destination plasmids for level Ω. All pDGB vectors incorporate a LacZ selection cassette flanked by four Type IIs restriction sites (BsaI, BsmBI), but positioned in inverted positions and orientations. To facilitate the visualization of the design, we assigned each 4 bp cleavage sequence a different label: those produced by BsaI digestion are labeled with squares and named with Arabic numbers (1,2,3), whereas BsmBI 4 bp cleavage sites are encircled and named with capital letters (A,B,C).