BIOINORGANIC CHEMISTRY A Short Course Second Edition

256 GROUP I AND II METALS IN BIOLOGICAL SYSTEMS: GROUP II

In the PDB: 1U6B structure the 3 ′ - exon is held in place through interaction
with the internal guide sequence (IGS) in helix P10 (PDB: 1U6B residues α G1,
G5, C6, C7, G8, and U9). The 5 ′ - exon 3 - residue fragment is also held in place
by IGS sequence G10, U11 and G12. The dt − 1 residue in the cadt − 1 nucleotide
forms a wobble base mispair with G10 of the IGS, which is in turn recognized
by the conserved A - rich J4/5 – J5/4 symmetric loop region. The hydrogen
bonding between the IGS sequence G10, U11, and G12 and the cadt − 1 nucleo-

Figure 6.7 A bacterial self - splicing group I intron with both exons (PDB: 1U6B).
Visualized using CambridgeSoft Chem3D Ultra 10.0 with notations in ChemDraw
Ultra 10.0.(Printed with permission of CambridgeSoft Corporation.) (See color
plate)

da+1

M 1

potassium ions

magnesium ions

dt-1 ΩG in spacefill

P2 tetraloop

(stick format)

P8 receptor

U1A loop

gray

P6

greenP3 green

P8

blue

P7

blue

P4

green

P9.0

blue

P9

blue

P5

green

P10

orange

P1

orange

P2

orange