NAME

       sigma - Simple greedy multiple alignment of non-coding DNA sequences

SYNOPSIS

       sigma [options] [inputfile.fasta] [inputfile2.fasta ...]

       Each fasta file may contain a single sequence or multiple sequences; all sequences will be
       aligned together.

DESCRIPTION

       Sigma ("Simple greedy multiple alignment") is an alignment program with a new algorithm
       and scoring scheme designed specifically for non-coding DNA sequence. It uses a strategy
       of seeking the best possible gapless local alignments, at each step making the best
       possible alignment consistent with existing alignments, and scores the significance of the
       alignment based on the lengths of the aligned fragments and a background model which may
       be supplied or estimated from an auxiliary file of intergenic DNA. With real data, while
       "correctness" can't be directly quantified for the alignment, running the PhyloGibbs motif
       finder on pre-aligned sequence suggests that Sigma's alignments are superior.

OPTIONS

       -A --aligned_output
          Aligned, pretty-printed output (compare with -F option) (default: only this). See also
          -C.

       -b --bgprobfile filename
          Auxiliary file (in fasta format) from which to read background sequences (overridden by
          -B). Typically this is a file containing large quantities of similar non-coding
          sequence, from which background probabilities of single- and di-nucleotides may be
          estimated.

       -B --bgseqfile filename
          File containing background probabilities. The format is described further below.

       -C --caps_only
          Use only upper-case letters in output sequence, for compatibility with output of some
          other programs like ClustalW and MLagan. By default, output is mixed-case (as in
          Dialign), and lower-case bases are treated as not aligned.

       -F --fasta_output
          Multi-fasta output (can use both -A and -F in either order). See also -C.

       -n --ncorrel number
          Background correlation (default 2=dinucleotide; 1=single-site basecounts, 0=0.25 per
          base).

       -x, --significance number
          Set limit for how probable the match is by chance (default 0.002, smaller=more
          stringent).

       -h, --help
          Displays this list of options.

MORE HELP

       The "significance" parameter (-x) determines whether local alignments are accepted or
       rejected. The default at present is 0.002. Experiments on synthetic data (described in the
       paper) suggest that 0.002 is about the threshold where sigma fails to align
       phylogenetically-unrelated data that has moderate (yeast-like) dinucleotide correlation.

       Using a “background model” appropriate to the sequences being aligned greatly reduces
       spurious alignments on synthetic data (and, one hopes, on real data too). The simplest way
       to ensure this is to supply, via the -b parameter, a FASTA-format file containing large
       quantities of similar sequence data (eg, if one is aligning yeast sequences, supply a file
       containing all intergenic yeast sequence).

       Instead of this, if the single-site and dinucleotide frequencies are known already, they
       may be supplied in a file via the -B option. The file format should be: one entry per
       line, with the mononucleotide or dinucleotide (case-insensitive) followed by the
       frequency. (eg, "A 0.3", "AT 0.16", etc on successive lines.) A sample file is in the
       "Background" subdirectory of the source distribution (on Debian systems, this file can be
       found in the /usr/share/doc/sigma-align/Background directory). A file like
       "yeast.nc.3.freq" in the "tests" subdirectory of the MEME source distribution works fine
       (trinucleotide counts are ignored).

REFERENCE

       Please cite Sigma: Rahul Siddharthan (2006) Multiple alignment of weakly-conserved
       non-coding DNA sequence BMC Bioinformatics 2006, 7:143 doi:10.1186/1471-2105-7-143
       Published 16 March 2006, available online at http://www.biomedcentral.com/1471-2105/7/143/

AUTHORS

       Rahul Siddharthan <rsidd@imsc.res.in>
          Wrote sigma. If you're using Sigma for actual research, please let the author know so
          that he can alert you of bugfixes or new releases.

       Charles Plessy <charles-debian-nospam@plessy.org>
          Wrote the manpage in DocBook XML for the Debian distribution.

COPYRIGHT

       Copyright © 2006-2007 Rahul Siddharthan
       Copyright © 2006-2007 Charles Plessy

       Sigma is free software. You can redistribute it and/or modify it under the terms of the
       GNU General Public License as published by the Free Software Foundation.

       On Debian systems, the complete text of the GNU General Public License can be found in
       /usr/share/common-licenses/GPL.