Primary Structure Section

The primary structure section of a PDB file contains the sequence of residues in each chain of the macromolecule. Embedded in these records are chain identifiers and sequence numbers that allow other records to link into the sequence.

The changes in the records in this section result from the standardization of nomenclature the standard nucleotides and nucleotide modifications.

SEQRES

SEQRES records contain the amino acid or nucleic acid sequence of residues in each chain of the macromolecule that was studied.

The ribo- and deoxyribonucleotides in the SEQRES records are now distinguished. The deoxy- forms of these residues are now identified with the residue names DA, DC, DG, DT, and DU. Modified nucleotides in the sequence are now identified by separate 3-letter residue codes. The use of the plus character prefix to label modified nucleotides (e.g. +A, +C, +T) is no longer used.

MODRES

SEQRES records contain the amino acid or nucleic acid sequence of residues in each chain of the macromolecule that was studied.

The ribo- and deoxyribonucleotides in the SEQRES records are now distinguished. The deoxy- forms of these residues are now identified with the residue names DA, DC, DG, DT, and DU. Modified nucleotides in the sequence are now identified by separate 3-letter residue codes. The use of the plus character prefix to label modified nucleotides (e.g. +A, +C, +T) is no longer used.