SOS: a simple interactive program for ab initio oligonucleotide sequencing by mass spectrometry.

Mass spectra of oligonucleotides derived from collision-induced dissociation following electrospray ionization provide an effective means of sequence determination, at the 20-mer level and below. An interactive, stand-alone computer program, Simple Oligonucleotide Sequencer (SOS) has been developed for rapid oligonucleotide sequencing from mass spectra, under user control on a residue by residue basis. Modifications can be defined in any combination for the base, sugar or backbone. Sequence ladders can be independently constructed in both the 5' --> 3' directions and 3' --> 5' directions, and graphically compared for homology and overlap. A particular advantage of this method is the ability to easily erase and rebuild alternate subsequences. The program can be used for ab initio sequencing of modified or unmodified oligonucleotides, for rapid verification of sequence, and in studies of fragmentation processes of model oligonucleotide derivatives.