A starting structure of suitable sequence is a prerequisite for many applications, including downstream use in X-ray crystallography, NMR, and molecular dynamics (MD) simulations. Browsing through the literature, I’ve noticed the following tools for such a purpose.

• The make-na server, a web-based automated tool for making nucleic acid helices powered by NAB. It supports abasic sites via the underscore character (_). According to the help page, “The structure file represents the abasic as the 3-letter code ‘3DR’ in DNA strands and ‘ N’ in RNA strands. These are Protein Data Bank conventions.” An example input is shown below:

    ATACCGATACG_TAGAC
      TG_CTATGCTATCTGT_

• The NAB itself, and the standalone fd_helix.c program which supports 6 fiber-based models of DNA or RNA.

• The NUCGEN program from the Bansal group. “The NUCGEN software generates double helical models with the backbone fixed in B-form DNA, but with appropriate modifications in the input data, it can also generate A-form DNA and RNA duplex structures.”

• 3DNA and its web interface. The ‘rebuild’ program can be used for constructing customized, single or duplex DNA/RNA structures based on a set of base-pair and step (helical) parameters. Moreover, the sugar-phosphate backbone in A-, B- or RNA conformation is allowed. The ‘fiber’ program incorporates a comprehensive list of 56 regular models, based mostly on fiber diffraction data. The list includes single, duplex, triplex, quadruplex, DNA, RNA structures or their hybrids. Notable, the classic Pauling’s triplex model is also available. The 3DNA web 2.0 makes these model-building features readily accessible to a large user base.

Overall, each of the tools listed above has its unique features and may fit better for different applications. It is to the benefit of the user community to have a choice.