RNAfold介绍

参考 ViennaRNA 与 iFrei.cn

RNAfold是Vienna RNA Package里的一种通过计算最小自由能(MFE:minimum free energy)来预测RNA二级结构的软件。它的算法和理论支持主要来源于M. Zuker和P. Stiegler在1981年发表的这篇Optimal computer folding of large RNA sequences using thermodynamic and auxiliary information.想了解Michael Zuker教授小组的一系列研究情况和MFE算法计算依据的可以点这里。

Vienna RNA Package的下载地址是http://www.tbi./~ivo/RNA/index.html，包括linux和windows两个版本。

RNAfold各个参数的含义见这里http://www.tbi./~ivo/RNA/man/RNAfold.html

这里举一个在linux下执行命令的例子：

解释下这里三个参数的意思。-noLP表示不允许有孤立碱基对，-noPS表示不生成结构图（只计算结果），d1指的是”dangling end”(悬挂端)的能量计算按照d1模式（有且仅有未配对的碱基可以参与最多一个悬挂端）。返回值就是“点-弧”结构图和预测的能量值（本例为-2.5）.

关于dangling end(dangling bases)的中文翻译在国内未能查到，这里暂翻译为“悬挂端”。可以参考这篇Long RNA Dangling End Has Large Energetic Contribution to Duplex Stability,dangling end指的就是在双链中未配对、会参与旁边“链栈”(stacks)能量增益的碱基。不同的悬挂组合产生的不同的能量增益，这在RNAfold计算中也会被考虑（寻求最好的dangle）。

图片来源：http://pubs./doi/abs/10.1021/ja0255406

RNAfold对dangling end的处理有两点：1.不确定的dangling end如果对相邻两侧的螺旋体都产生影响的话将被计算；2.在MFE预测后能量值将被修正。

附wikipedia上的词条：List of RNA structure prediction software

RNAfold帮助文档：

NAME

RNAfold - calculate secondary structures of RNAs

SYNOPSIS

RNAfold [-p[0|2]] [-C] [-T temp] [-4] [-d[0|1|2|3]] [-noLP] [-noGU] [-noCloseGU] [-e 1|2] [-P paramfile] [-nsp pairs] [-S scale] [-circ] [-MEA [gamma]]

DESCRIPTION

RNAfold reads RNA sequences from stdin, calculates their minimum free energy (mfe) structure and prints to stdout the mfe structure in bracket notation and its free energy. If the -p option was given it also computes the partition function (pf) and base pairing probability matrix, and prints the free energy of the thermodynamic ensemble, the frequency of the mfe structure in the ensemble, and the ensemble diversity to stdout.

It also produces PostScript files with plots of the resulting secondary structure graph and a "dot plot" of the base pairing matrix. The dot plot shows a matrix of squares with area proportional to the pairing probability in the upper right half, and one square for each pair in the minimum free energy structure in the lower left half. For each pair i-j with probability p>10E-6 there is a line of the form
i j sqrt(p) ubox
in the PostScript file, so that the pair probabilities can be easily extracted.

Sequences are read in a simple text format where each sequence occupies a single line. Each sequence may be preceded by a line of the form
> name
to assign a name to the sequence. If a name is given in the input
 PostScript files "name_ss.ps" and "name_dp.ps" are produced for the structure and dot plot, respectively. Otherwise the file names default to rna.ps and dot.ps. Existing files of the same name will be overwritten.
The input format is similar to fasta except that even long sequences may not be interrupted by line breaks, and the header lines are optional. The program will continue to read new sequences until a line consisting of the single character @ or an end of file condition is encountered.

OPTIONS

• -p

• Calculate the partition function and base pairing probability matrix in addition to the mfe structure. Default is calculation of mfe structure only. In addition to the MFE structure we print a coarse representation of the pair probabilities in form of a pseudo bracket notation, followed by the ensemble free energy, as well as the centroid structure derived from the pair probabilities together with its free energy and distance to the ensemble. Finally it prints the frequency of the mfe structure, and the structural diversity (mean distance between the structures in the ensemble). See the description of pf_fold() and mean_bp_dist() and centroid() in the RNAlib documentation for details.
  Note that unless you also specify -d2 or -d0, the partition function and mfe calculations will use a slightly different energy model. See the discussion of dangling end options below.

• -p0

• Calculate the partition function but not the pair probabilities, saving about 50% in runtime. Prints the ensemble free energy -kT ln(Z).

• -p2

• In addition to pair probabilities compute stack probabilities, i.e. the probability that a pair (i,j) and the immediately interior pair (i+1,j-1) are formed simultaneously. A second postscript dot plot called "name_dp2.ps", or "dot2.ps" (if the sequence does not have a name), is produced that contains pair probabilities in the upper right half and stack probabilities in the lower left.

• -MEA [gamma]

• Calculate an MEA (maximum expected accuracy) structure, where the expected accuracy is computed from the pair probabilities: each base pair (i,j) gets a score 2*gamma*p_ij and the score of an unpaired base is given by the probability of not forming a pair. The parameter gamma tunes the importance of correctly predicted pairs versus unpaired bases. Thus, for small values of gamma the MEA structure will contain only pairs with very high probability. The default value is gamma=1. Using -MEA implies -p for computing the pair probabilities.

• -C

• Calculate structures subject to constraints. The program reads first the sequence, then a string containing constraints on the structure encoded with the symbols: | (the corresponding base has to be paired x (the base is unpaired) < (base i is paired with a base j>i) > (base i is paired with a base j<i) and matching brackets ( ) (base i pairs base j) With the exception of "|", constraints will disallow all pairs conflicting with the constraint. This is usually sufficient to enforce the constraint, but occasionally a base may stay unpaired in spite of constraints. PF folding ignores constraints of type "|".

• -T temp

• Rescale energy parameters to a temperature of temp C. Default is 37C.

• -4

• Do not include special stabilizing energies for certain tetra-loops. Mostly for testing.

• -d[0|1|2|3]

• How to treat "dangling end" energies for bases adjacent to helices in free ends and multi-loops: With (-d1) only unpaired bases can participate in at most one dangling end, this is the default for mfe folding but unsupported for the partition function folding. With -d2 this check is ignored, dangling energies will be added for the bases adjacent to a helix on both sides in any case; this is the default for partition function folding (-p).-d or -d0 ignores dangling ends altogether (mostly for debugging).
  With -d3 mfe folding will allow coaxial stacking of adjacent helices in multi-loops. At the moment the implementation will not allow coaxial stacking of the two interior pairs in a loop of degree 3 and works only for mfe folding.
  Note that by default (as well as with -d1 and -d3) pf and mfe folding treat dangling ends differently. Use -d2 in addition to -p to ensure that both algorithms use the same energy model.

• -noLP

• Produce structures without lonely pairs (helices of length 1). For partition function folding this only disallows pairs that canonly occur isolated. Other pairs may still occasionally occur as helices of length 1.

• -noGU

• Do not allow GU pairs.

• -noCloseGU

• Do not allow GU pairs at the end of helices.

• -e 1|2

• Rarely used option to fold sequences from the artificial ABCD... alphabet, where A pairs B, C-D etc. Use the energy parameters for GC (-e 1) or AU (-e 2) pairs.

• -P <paramfile>

• Read energy parameters from paramfile, instead of using the default parameter set. A sample parameter file should accompany your distribution. See the RNAlib documentation for details on the file format.

• -nsp pairs

• Allow other pairs in addition to the usual AU,GC,and GU pairs.pairs is a comma separated list of additionally allowed pairs. If a the first character is a "-" then AB will imply that AB and BA are allowed pairs. e.g. RNAfold -nsp -GA will allow GA and AG pairs. Nonstandard pairs are given 0 stacking energy.

• -S scale

• In the calculation of the pf use scale*mfe as an estimate for the ensemble free energy (used to avoid overflows). The default is 1.07, useful values are 1.0 to 1.2. Occasionally needed for long sequences.

• -circ

• Assume a circular (instead of linear) RNA molecule.

• -noPS

• Do not produce postscript drawing of the mfe structure.

REFERENCES

The calculation of mfe structures is based on dynamic programming algorithm originally developed by M. Zuker and P. Stiegler. The partition function algorithm is based on work by J.S. McCaskill. The energy parameters are taken from:
D.H. Mathews, J. Sabina, M. Zuker and H. Turner "Expanded Sequence Dependence of Thermodynamic Parameters Provides Robust Prediction of RNA Secondary Structure" JMB, 288, pp 911-940, 1999
A. Walter, D Turner, J Kim, M Lyttle, P M[:u]ller, D Mathews, M Zuker "Coaxial stacking of helices enhances binding of oligoribonucleotides.." PNAS, 91, pp 9218-9222, 1994

If you use this program in your work you might want to cite:

I.L. Hofacker, W. Fontana, P.F. Stadler, S. Bonhoeffer, M. Tacker, P. Schuster (1994) Fast Folding and Comparison of RNA Secondary Structures. Monatshefte f. Chemie 125: 167-188
M. Zuker, P. Stiegler (1981) Optimal computer folding of large RNA sequences using thermodynamic and auxiliary information, Nucl Acid Res 9: 133-148
J.S. McCaskill (1990) The equilibrium partition function and base pair binding probabilities for RNA secondary structures, Biopolymers 29: 1105-1119
I.L. Hofacker & P.F. Stadler (2006) Memory Efficient Folding Algorithms for Circular RNA Secondary Structures, Bioinformatics
A.F. Bompf??newerer, R. Backofen, S.H. Bernhart, J. Hertel, I.L. Hofacker, P.F. Stadler, S. Will (2007) "Variations on RNA Folding and Alignment: Lessons from Benasque" J. Math. Biol.
D. Adams (1979) The hitchhiker's guide to the galaxy, Pan Books, London

VERSION

This man page documents version 1.8.5 Vienna RNA Package.

AUTHORS

Ivo L Hofacker, Walter Fontana, Sebastian Bonhoeffer, Peter F Stadler.

BUGS

If in doubt our program is right, nature is at fault. Comments should be sent to rna@tbi..