5.17 Mutation

Mutations are available on a 1-by-1 basis using the graphics. After selecting “Mutate...” from the “Model/Fit/Refine” dialog, click on an atom in the graphics. A “Residue Type” window will now appear. Select the new residue type you wish and the residue in the graphics is updated to the new residue type ¹. The initial position of the new rotamer is the a priori most likely rotamer. Note that in interactive mode, such as this, a residue type match ² will not stop the mutation action occurring.

5.17.1 Mutating DNA/RNA

Mutation of DNA or RNA can be performed using “Simple Mutate” from the Model/Fit/Refine dialog. Residues need to be named "Ad", "Gr", "Ur" etc.

5.17.2 Multiple mutations

This dialog can be found under Calculate -> Mutate Residue Range. A residue range can be assigned a sequence and optionally fitted to the map. This is useful converting a poly-ALA model to the correct sequence ³.

Multiple mutations are also supported via the scripting interface. Unlike the single residue mutation function, a residue type match will prevent a modification of the residue ⁴. Two functions are provided: To mutate a whole chain, use (mutate-chain imol chain-id sequence) where:

chain-id is the chain identifier of the chain that you wish to mutate (e.g. "A") and

imol is molecule number.

sequence is a list of single-letter residue codes, such as "GYRESDF" (this should be a straight string with no additional spaces or carriage returns).

Note that the number of residues in the sequence chain and those in the chain of the protein must match exactly (i.e. the whole of the chain is mutated (except residues that have a matching residue type).)

To mutate a residue range, use

• (mutate-residue-range imol chain-id start-res-no stop-res-no sequence)

where

start-res-no is the starting residue for mutation

stop-res-no is the last residue for mutation, i.e. using values of 2 and 3 for start-res-no and stop-res-no respectively will mutate 2 residues.

Again, the length of the sequence must correspond to the residue range length. Note also that this is a protein sequence - not nucleic acid.

For mutation of nucleic acids, use:

(mutate-nucleotide-range imol chain-id resno-start resno-end sequence)

5.17.3 Mutating to a Non-Standard Residue

Sometimes one might like to model post-translational or other such modifications. How is that done, if the new residue type is not one of the standard residue types?

There is a scripting function:

(mutate-by-overlap imol chain-id resno new-three-letter-code)

This imports a model residue for the new residue type and overlays it on to the given residue by using graph-matching to determine the equivalent atoms.

The GUI for this can be found under Extensions -> Modelling -> Replace Residue... (for this to work, you need to be centred on the residue you wish to replace).

Note that if you are replacing are conventional protein residue with a modified form (e.g. replacing a TYR with a phoso-tyrosine or a LYS with an acetyl-lysine) you will need to make sure that the group of the resulting restraints is an L-peptide (use Edit -> Restraints to check and modify the restraints group. Likewise for modified RNA/DNA nucleotides, you need to specify the group as RNA or DNA as appropriate.

5.17.4 Mutate and Autofit

The function combines Mutation and Auto Fit Rotamer and is the easiest way to make a mutation and then fit to the map. You can currently only “Mutate and Autofit” protein residues (i.e. things with a rotamer dictionary.

5.17.5 Renumbering

Renumbering is straightforward using the renumber dialog available under Calculate -> Renumber Residue Range.... There is also a scripting interface:

(renumber-residue-range imol chain-id start-res-no last-resno offset)