MFold predicts optimal and suboptimal secondary structures for an RNA or DNA molecule using the most recent energy minimization method of Zuker.
MFold is an adaptation of the mfold package (version 2.3) by Zuker and Jaeger that has been modified to work with the Wisconsin Package(TM). Their method uses the energy rules developed by Turner and colleagues to determine optimal and suboptimal secondary structures for an RNA molecule and the energy rules compiled and developed by SantaLucia and colleagues to determine optimal and suboptimal secondary structures for a single-stranded DNA molecule. (See the ACKNOWLEDGEMENTS topic for references.)
Using energy minimization criteria, any predicted "optimal" secondary structure for an RNA or DNA molecule depends on the model of folding and the specific folding energies used to calculate that structure. Different optimal foldings may be calculated if the folding energies are changed even slightly. Because of uncertainties in the folding model and the folding energies, the "correct" folding may not be the "optimal" folding determined by the program. You may therefore want to view many optimal and suboptimal structures within a few percent of the minimum energy. You can use the variation among these structures to determine which regions of the secondary structure you can predict reliably. For instance, a region of the RNA molecule containing the same helix in most calculated optimal and suboptimal secondary structures may be more reliably predicted than other regions with greater variation.
MFold calculates energy matrices that determine all optimal and suboptimal secondary structures for an RNA or DNA molecule. The program writes these energy matrices to an output file. A companion program, PlotFold, reads this output file and displays a representative set of optimal and suboptimal secondary structures for the molecule within any increment of the computed minimum free energy you choose. You can choose any of several different graphic representations for displaying the secondary structures in PlotFold.
The output file produced by MFold contains the calculated energy matrices that determine all optimal and suboptimal secondary structures for the folded nucleic acid molecule. You cannot read the output file produced by MFold. This file is read by the companion program, PlotFold, which can display any of several different graphic representations of optimal and suboptimal secondary structures for the folded molecule.
MFold accepts a single nucleotide sequence as input. If MFold rejects your nucleotide sequence, turn to Appendix VI to see how to change or set the type of a sequence.
MFold predicts optimal and suboptimal secondary structures for an RNA or DNA molecule using the most recent energy minimization method of Zuker. PlotFold displays the optimal and suboptimal secondary structures for an RNA or DNA molecule predicted by MFold.
StemLoop finds all possible stems (inverted repeats) above some minimum quality that you can set, but StemLoop cannot recognize a structure with gaps (bulge loops or uneven bifurcation loops). The stems can be plotted with DotPlot.
The general algorithm for determining multiple optimal and suboptimal secondary structures is described by the author of the program, Dr. Michael Zuker (Science 244, 48-52 (1989)). A description of the folding parameters used in the algorithm is presented in Jaeger, Turner, and Zuker (Proc. Natl. Acad. Sci. USA, 86, 7706-7710 (1989)).
You may want to constrain the computed foldings to require specific helices and/or unpaired regions based on experimental data.
You can insist that all optimal and suboptimal foldings include a specified helix. (This is equivalent to the double force option in Zuker's original version of the program.) To do this, specify the first base pair, between bases i and j, and the length of the helix, k, using -FORCe1=i,j,k. This forces base pairs s(i)-s(j), s(i+1)-s(j-1),..., s (i+k-1)-s(j-k+1).
You can insist that a group of consecutive bases be double-stranded without specifying the pairing partner for each base. (This is equivalent to the single force option in Zuker's original version of the program.) To do this, specify the first base of the forced region, i, and the length of the forced region, k, using -FORCe1=i,0,k. The 0 between i and k is necessary to tell the program that you are forcing a group of contiguous bases to be double-stranded, rather than forcing a specific helix. This forces bases s(i), s(i+1),..., s(i+k-1) to be double-stranded.
You can force up to eight additional regions to pair with -FORCe2=l,m,n ... -FORCe9=x,y,z.
The only allowable base pairs are A-T/U, G-C, and G-T/U. If you force other base pairing, the program ignores them.
You can prevent a specified helix from forming in all optimal and suboptimal foldings. (This is equivalent to the double prevent option in Zuker's original version of the program.) To do this, specify the first base pair of the helix you want to prevent, between bases i and j, and the length of the helix, k, using -PREVent1=i,j,k. This prevents the helix containing base pairs s(i)-s(j), s(i+1)-s(j-1),..., s(i+k-1)-s(j-k+1) from forming. Only a specific, single helix is prevented; the prevented bases are still free to participate in other helices. You can prevent a group of consecutive bases from being involved in any helix, forcing them to remain single-stranded in all predicted foldings. (This is equivalent to the single prevent option in Zuker's original version of the program.) To do this, specify the first base of the single-stranded region, i, and the length of the single-stranded region, k, using -PREVent1=i,0,k. The 0 between i and k is necessary to tell the program that you are forcing a single-stranded region, rather than preventing a specific helix from forming. This will force bases s(i), s(i+1),..., s(i+k-1) to be single-stranded.
You can prevent up to eight additional regions from pairing with -PREVent2=l,m,n ... -PREVent9=x,y,z.
You can exclude a region of the RNA molecule from folding when a secondary structure model for that region already exists. (This is equivalent to the closed excision option in Zuker's original version of the program.) To do this, specify the base pair that closes off the excluded region, between bases i and j, using -CLOSedexcise1=i,j. MFold folds the remainder of the sequence, including the base pair between i and j. The only allowable base pairs are A-T/U, G-C, and G-T/U. Attempts to force other base pairing produce undefined results. You can specify up to eight additional regions for closed excisions with -CLOSedexcise2=k,l ... -CLOSedexcise9=y,z.
You can also exclude a region of the RNA molecule from participating in a secondary structure as if that region were spliced from the molecule before folding. (This is equivalent to the open excision option in Zuker's original version of the program.) To do this, specify the beginning and ending base numbers, i and j respectively, of the excluded region using -OPENexcise1=i,j. The region from i to j, inclusive, is removed and base i-1 is "ligated" to j+1 before folding the molecule. You can specify up to eight additional regions for open excisions with -OPENexcise2=k,l ... -OPENexcise9=y,z.
See RESTRICTIONS for constraints in plotting the secondary structures of RNA molecules in which a region has be excluded from folding with either -CLOSedexcise or -OPENexcise.
If you want to specify multiple regions for any folding constraint discussed above, you must number that constraint sequentially. For instance, if you want to specify two excluded regions for open excisions, you would need to specify -OPENexcise1 and -OPENexcise2; specifying -OPENexcise1 and -OPENexcise3 would cause the program to recognize only the first excluded region.
If you don't specify any folding constraints as described above, yet an optimal folding is inconsistent with the experimental data, then one of the predicted suboptimal foldings may be consistent.
A maximum of 1400 bases can be folded.
Sequences should only contain the symbols A, C, G, and T/U.
If you exclude a region of the RNA or DNA molecule from folding with either -CLOSedexcise or -OPENexcise, you can display the predicted secondary structures using only the text output option in PlotFold; do not use any of the graphic plotting options of PlotFold to display the results.
MFold does not predict RNA secondary structures containing pseudoknots.
There are several differences between the GCG implementation of MFold and Dr. Zuker's mfold package. Dr. Zuker's lrna and crna programs, which fold linear and circular sequences, respectively, are combined into a single GCG program. By default, MFold treats the input sequence as a linear molecule. To fold a circular sequence, use Folds a circular RNA molecule.
In Dr. Zuker's original implementation, the program takes a nucleic acid sequence as input, computes the energy matrices, and then displays representations of optimal and suboptimal secondary structures. Dr. Zuker's program allows you the option of storing the energy matrices in a save run of the program and later displaying the secondary structures in a separate continue run. The GCG version of MFold always saves the energy matrices into an output file. A separate program, PlotFold, reads these energy matrices and displays representative secondary structures. Depending on the size of the RNA sequence, the file containing the energy matrices can be very large. For example, the output file created in the MFold example session requires approximately 0.35 megabytes of disk storage. You should consider deleting files that you no longer need.
The default energy files are used by the program to predict folding at 37(o)C. Dr. Zuker's newtemp program allows you to generate energy files for folding nucleic acid molecules at any temperature between 0(o)C and 100(o)C. The GCG version of MFold does not require separate energy files for folding at another temperature. You can specify another folding temperature by using Folding temperature (Celsius) set to 45 (to fold at 45(o)C, for example).
In Dr. Zuker's original implementation, the symbols B, Z, H, and V/W represent, respectively, the bases A, C, G, and U that are accessible to single-strand nuclease cleavage. The GCG version of MFold does not recognize these symbols as nuclease-sensitive bases; sequences should only contain the symbols A, C, G, and T/U.
GCG is licensed to distribute MFold by the National Research Council of Canada. If you use MFold for published research, please cite Dr. Zuker's Science paper (reference below). We are very grateful to Dr. Zuker both for making his work available to GCG and for helping us incorporate his work into the Wisconsin Package.
MFold is an adaptation of the mfold package (version 2.3) by Zuker and Jaeger (Zuker, M. (1989). Science 244, 48-52; Jaeger, J.A. Turner, D.H., and Zuker, M. (1989). Proc. Natl. Acad. Sci. USA, 86, 7706-7710; Jaeger, J.A., Turner, D.H., and Zuker, M. (1990). In Methods in Enzymology, 183, 281-306) that has been modified to work with the Wisconsin Package. Their method uses the energy rules developed by Turner and colleagues (Freier, S.M., Kierzek, R., Jaeger, J.A., Sugimoto, N., Caruthers, M.H., Neilson, T., and Turner, D.H. (1986). Proc. Natl. Acad. Sci. USA 83, 9373-9377; Turner, D.H. Sugimoto, N., Jaeger, J.A., Longfellow, C.E., Freier, S.M. and Kierzek, R. (1987). Cold Spring Harbor Symp., Quant. Biol. 52, 123-133; Turner, D.H., Sugimoto, N., and Freier, S.M. (1988). Annu. Rev. Biophys. Biophys. Chem. 17, 167-192) to determine optimal and suboptimal secondary structures for an RNA molecule.
The energy rules to determine optimal and suboptimal secondary structures for a single-stranded DNA molecule were developed and compiled by Dr. John SantaLucia, Jr. and colleagues. If you use MFold to predict single-stranded DNA secondary structures, please cite the following references: SantaLucia, J.Jr. (1998). Proc. Natl. Acad. Sci. USA 95, 1460-1465; SantaLucia, J.Jr. and Allawi, H.T. (1977). Biochemistry 36, 10581-10594.
MFold was modified to work with version 7.2 of the Wisconsin Package by Irv Edelman.
You can set the parameters listed below from the command line. For more information, see "Using Program Parameters" in Chapter 3, Using Programs in the User's Guide.