PRATT version 2.1 Documentation

Written by:
Inge Jonassen,

Created: February 7th 1997

New features in version 2.1:

2.0 was the last major version of Pratt, in the 2.1 version only minor things have been added/changed. A few bugs in version 2.0 have been fixed in 2.1. Inge Jonassen is grateful to Dr. John F. Collins, University of Edinburgh, who proposed many of the added features, and discovered many of the bugs.

Command line search control -- the user can now choose values for all parameters that are in the menu directly from the command line. This makes it quicker for experienced user to specify his/her search, and also makes it easier to call Pratt from inside other programs.
When showing the sequence segments matching each pattern, the sequence symbols matching non-wildcard positions (components) in the pattern, are written in upper-case while sequence symbols matching wild-cards are in lower-case. Also, gaps (-) are added to align the symbols matching each pattern component.
On-line help is available from the menu by typing "help <option>" where option is one of the options in the menu or help for general help about Pratt.
Summary information about where the patterns match in the sequences is written horizontally or vertically.
The user can restrict where in the sequences patterns should be looked for. This can be useful for example if the user knows some constraints on the position of the patterns in one or more of the sequences.
When using Pratt interactively (using the menu), some summary information about the search parameters will be shown after the user asks the search to be started, and the user is given the opportunity to go back to the menu to change parameter values.

What was new in version 2.0?

In addition to the set S of protein sequences, the user can input:

A multiple sequence alignment of some of the sequences in S, or of all sequences in S, and Pratt will search only for patterns consistent with the alignment.
Input a special query sequence, and Pratt will search only for patterns matching this sequence and some propotion of the other sequences.

Branch-and-bound and heuristics have been added to make the pattern search more efficient. The speed-up is significant, in particular for sets of relatively similar (closely related) sequences.

Pratt has integrated new pattern scoring mechanisms, including one taking into account the diversity of the sequences matching a pattern, and another taking into account the number of sequences matching each pattern.

1. What is Pratt?

Pratt is a tool that allows the user to search for patterns conserved in a set of protein sequences. The user can specify what kind of patterns should be searched for, and how many sequences should match a pattern to be reported.

1.1 References:

Finding flexible patterns in unaligned protein sequences.
Inge Jonassen, John F. Collins, Desmond Higgins.
Protein Science 1995;4(8):1587-1595.
Efficient discovery of conserved patterns using a pattern graph.
Inge Jonassen.
Submitted to CABIOS.
Scoring function for pattern discovery programs taking into account sequence diversity.
Inge Jonassen, Carsten Helgesen, Desmond Higgins.
Dept. of Informatics, Univ. of Bergen, Reports in Informatics no 116, Febr. 1996.
Discovering patterns and subfamilies in biosequences.
A. Brazma, I. Jonassen, E. Ukkonen, and J. Vilo.
in Proceedings of the Fourth International Conference on Intellignent Systems for Molecular Biology (ISMB-96), AAAI Press 1996, p 34-43.

1.2 Pattern terminology

Pratt is able to discover conserved patterns in the sequences.

The patterns that can be found is a subset of the set of patterns that can be described using Prosite notation.

A pattern that can be found by Pratt can be written on the form

A(1)-x(i1,j1)-A2-x(i2,j2)-....A{p-1}-x(i{p-1},j{p-1})-Ap

where

A(k) is a component of the pattern, either specifying one amino acid, e.g. C, or a set of amino acids, e.g. [ILVF].
A pattern component A(k) is an

identity component
if it specifies exactly one amino acid (for instance C or L),
ambiguous component
if it specifies more than one (for instance [ILVF] or [FWY]).
i(k), j(k) are integers so that i(k)<=j(k) for all k. The part x(ik,jk) specifies a wildcard region of the pattern matching between ik and jk arbitrary amino acids.
A wildcard region x(ik,jk) is

flexible
if jk is bigger than ik (for example x(2,3). The flexibility of such a region is jk-ik.br> For example the flexibility of x(2,3) is 1.
fixed
if j(k) is equal to i(k), e.g., x(2,2) which can be written as x(2).

The product of flexibility for a pattern is the product of the flexibilities of the flexible wildcard regions in the pattern, if any, otherwise it is defined to be one.

Examples:

C-x(2)-H is a pattern with two components (C and H) and one fixed wildcard region. It matches any sequence containing a C followed by any two arbitrary amino acids followed by an H. For example aaChgHyw and liChgHlyw.
C-x(2,3)-H is a pattern with two components (C and H) and one flexible wildcard region. It matches any sequence containing a C followed by any two or three arbitrary amino acids followed by an H. For example aaChgHywk and liChgaHlyw.
C-x(2,3)-[ILV] is a pattern with two components (C and [ILV]) and one flexible wildcard region. It matches any sequence containing a C followed by any two or three arbitrary amino acids followed by an I, L or V.

2. User manual

2.1 Format of input sequences.

Make a file or a set of files containing the set of sequences to be analysed. Currently, Pratt can read one of the formats:

Fasta format. One file containing all the sequences. One sequence is specified by
- one line starting with '>' in position 1 and then the name of the sequence, and
- some lines containging the sequence in upper or lower case. The end of a sequence is identified by looking for either the start of a new sequence or the end of the file.
- Pratt does not allow for annotation in FastA format input files.
SWISS-PROT format. One file containing all the sequences. One sequence is specified by
- one line starting with 'ID' and then the name of the sequence, followed by an arbitrary number of lines, and then
- a line starting with 'SQ' (rest of the line ignored), followed by the sequence (on one or several lines), followed by a line starting with '//'

2.2 Command line.

Command line:

    Pratt <format> <filename> [options]

where <format> is one of:

fasta
swissprot

and <filename> is the name of a file containing the sequences in the given format

2.3 Using the menu to control your search.

When you run Pratt, it will give you a menu allowing you to set a variety of parameters controlling:

what kind of patterns Pratt is going to look for,
how many sequences a pattern should match,
lower threshold on significance for a pattern to be reported, and
how many patterns should be reported.
greediness in search (new in version 2.0)
if a query sequence or an alignment is to be used (new in version 2.0)
if a pattern is restricted to match within a specified region in one or several of the sequences (new in version 2.1).

Schematic figure of algorithm used in Pratt version 2.x

Overview of the pattern discovery algorithm. The user inputs a set unaligned sequences, and the minimum number of sequences to match a pattern. (i): During this phase, patterns are constrained to the pattern class defined by the bounds set using the menu. A pattern graph can be constructed either from the shortest sequences in S (1), from a special query sequence (2), or from a multiple sequence alignment (3). A search is done for the highest scoring patterns in the class that can be derived from the pattern graph. The block data structure is used to find all matches to each pattern. (ii): The highest scoring patterns found during this search, are input to a heuristic pattern refinement algorithm, where more ambiguous pattern components (from a list given by the user in Pratt.sets) can be added to the patterns found during phase (i). The refinement phase is optional.

Sample run of Pratt version 2.1:


 ------------------------------------------------------------

                Pratt version 2.1, Sept. 1996

                 Written by Inge Jonassen, 

                   University of Bergen

                           Norway

                   email: inge@ii.uib.no

                 For more information, see 

            http://www.ii.uib.no/~inge/Pratt.html

------------------------------------------------------------

                        Please quote:

             I.Jonassen, J.F.Collins, D.G.Higgins.

             Protein Science 1995;4(8):1587-1595.

                         I.Jonassen

                     submitted to CABIOS 

------------------------------------------------------------

 

 

 

                Pratt version 2.1 

 

        Analysing 166 sequences from file snake

 

PATTERN CONSERVATION:

   CM: min Nr of Seqs to Match                166

   C%: min Percentage Seqs to Match         100.0

 

PATTERN RESTRICTIONS :

   PP: pos in seq [off,complete,start]        off

   PL: max Pattern Length                      50

   PN: max Nr of Pattern Symbols               50

   PX: max Nr of consecutive x's                5

   FN: max Nr of flexible spacers               2

   FL: max Flexibility                          2

   FP: max Flex.Product                        10

   BI: Input Pattern Symbol File              off

   BN: Nr of Pattern Symbols Initial Search    20

 

PATTERN SCORING:

   S: Scoring [info,mdl,tree,dist,ppv]       info

 

SEARCH PARAMETERS:

   G: Pattern Graph from [seq,al,query]       seq

   E: Search Greediness                         3

   R: Pattern Refinement                       on

   RG: Generalise ambiguous symbols           off

 

OUTPUT:

   OF: Output Filename              snake.166.pat

   OP: PROSITE Pattern Format                  on

   ON: max number patterns                     50

   OA: max number Alignments                   50

   M: Print Patterns in sequences              on

   MR: ratio for printing                      10

   MV: print vertically                       off

 

 

X: eXecute program

Q: Quit

 

help: for on-line help

 

Command:

C Options:

The C parameters control how many sequences a pattern should match to be considered by Pratt:

CM:: Set the minimum number of sequences to match a pattern. Pratt will only report patterns that match at least the chosen number of the sequences that you have input. Pratt will not allow you to choose a value higher than the number of sequences input.
C%:: set the minimum percentage of the input sequences that should match a pattern. If you set this to, say 80, Pratt will only report patterns matching at least 80 % of the sequences input.

G Options:

Allows the use of an alignment or a query sequence to restrict the pattern search.

If G is set to al or query, another option GF will appear allowing the user to give the name of a file containing a multiple sequence alignment (in Clustal W format), or a query sequence in FastA format (without annotation).n Only patterns consistent with the alignment/matching the query sequence will be considered.

Loosely a pattern is considered consistent with the alignment if

each symbol in the pattern (e.g. A) corresponds to a ungapped column in the alignment where all the characters match the pattern symbol (in the example, A).
the wildcards in the pattern are compatible with the number of residues between the corresponding columns in the alignment.

For instance the pattern A-x(2,3)-B is consistent with the alignment


   ALVGB

   AG-LB

   ALD-B

For more details see I. Jonassen
Efficient discovery of conserved patterns using a pattern graph.
Submitted to CABIOS

B Options:

Using the B options (BN,BI,BF) on the menu you can control which pattern symbols will be used during the initial pattern search and during the refinement phase. In the pattern C-x(2)-[DE], C and [DE] are the symbols.

The pattern symbols that can be used, are read from a file if the BI option is set, otherwise a default set will be used.

The default set has as the 20 first elements, the single amino acid symbols, and it also contains a set of ambiguous symbols, each containing amino acids that share some physio-chemical properties

If BI is set, option BF will appear to allow you to give the name of the file. In the file each symbol is given on a separate line contatining the letters that the symbol should match. For instance the file could be:


C

DE

and only patterns with the symbols C and [DE] would be considered. During the initial search, pattern symbols corresponding to the first BN lines can be used. Increasing BN will slow down the search and increase the memory usage, but allow more ambiguous pattern symbols.

P Options:

The P options are for controlling the patterns to be considered by Pratt. See also the F options for controlling flexibility.

Option PL:

allows you to set the maximum length of a pattern. The length of the pattern C-x(2,4)-[DE] is 1+4+1=6. The memory requirement of Pratt depends on L; a higher L value gives higher memory requirement.

Option PN:

using this you can set the maximum number of symbols in a pattern. The pattern C-x(2,4)-[DE] has 2 symbols (C and [DE]). When PN is increased, Pratt will require more memory.

Option PX:

Using this option you can set the maximum length of a wildcard. Examples of wildcards and lengths are


   x       -  1

  x(10)    - 10

 x(3,4)    -  4

Increasing PX will increase the time used by Pratt, and also slightly the memory required.

F Options:

The F options control flexible wildcards in the patterns:

Option FN:

Using this option you can set the maximum number of flexible wildcards (matching a variable number of arbitrary sequence symbols). For instance x(2,4) is a flexible wildcard, and the pattern


   C-x(2,4)-[DE]-x(10)-F

contains one flexible wildcard.

Increasing FN will increase the time used by Pratt.

Option FL:

you can set the maximum flexibility of a flexible wildcard (matching a variable number of arbitrary sequence symbols). For instance x(2,4) and x(10,12) has flexibility 2, and x(10) has flexibility 0. Increasing FL will increase the time used by Pratt.

Option FP:

Using option FP you can set an upper limit on the product of a flexibilities for a pattern. This is related to the memory requirements of the search, and increasing the limit, increases the memory usage. Some patterns and the corresponding product of flexibilities:


C-x(2,4)-[DE]-x(10)-F      - (4-2+1)*(10-10+1)= 3

C-x(2,4)-[DE]-x(10-14)-F   - (4-2+1)*(14-10+1)= 3*5= 15

Option E:

Using the E parameter you can adjust the greediness of the search. Setting E to 0 (zero), the search will be exhaustive. Increasing E increases the greediness, and decreases the time used in the search.

Option R:

When the R option is switched on, patterns found during the initial pattern search are input to a refinement algorithm where more ambiguous pattern symbols can be added.
For instance the pattern


    C-x(4)-D

might be refined to


C-x-[ILV]-x-D-x(3)-[DEF]

If the RG option is switched on, then ambiguous symbols listed in the symbols file (or in the default symbol set -- see help for option B), are used. If RG is off, only the letters needed to match the input sequences are inlcuded in the ambiguous pattern positions.
For example, if [ILV] is a listed allowed symbol, and [IL] is not, [IL] can be included in a pattern if RG is off, but if RG is on, the full symbol [ILV] will be included instead.

O Options:

The O options allow you to control the output from Pratt:

Option OF:: allows you to specify the name of the file to which Pratt will write its output
Option OP:: when switched on, patterns will be output in PROSITE style (for instance C-x(2,4)-[DE]). When switched off, patterns are output in a simpler consensus pattern style (for instance Cxx--[DE] where x matches exactly one arbitrary sequence symbol and - matches zero or one arbitrary sequence symbol).
Option ON:: set the max. nr of patterns to be found by Pratt.
Option OA:: set the max. nr of patterns for which Pratt is to produce an alignment of the sequence segments matching it.

M Options:

If the M option is set, then Pratt will print out the location of the sequence segments matching each of the (maximum 52) best patterns. The patterns are given labels A, B,...Z,a,b,...z in order of decreasing pattern score. Each sequence is printed on a line, one character per K-tuple in the sequence. If pattern with label C matches the third K-tuple in a sequence C is printed out. If several patterns match in the same K-tuple, only the best will be printed.

Option MR:: sets the K value (ratio) used for printing the summary information about where in each sequence the pattern matches are found.
Option MV:: if set, the output is printed vertically instead of horizontally, vertical output can be better for large sequence sets.

S Options:

The S option allows you to control the scoring of patterns. There are five possible scoring schemes to be used:

info: patterns are scored by their information content as defined in (Jonassen et al, 1995). Note that a pattern's score is independent of which sequences it matches.
mdl: patterns are scored by a Minimum Description Length principle derived scoring scheme, which is related to the one above, but penalises patterns scoring few sequences vs. patterns scoring many. Parameters Z0-Z3 appears when this scoring scheme is used.
tree: a pattern is scored higher if it contains more information and/or if it matches more diverse sequences. The sequence diversity is calculated from a dendrogram which has to be input.
dist: similar to the tree scoring, except a matrix with pairwise the similarity between all pairs of input sequences are used instead of the tree. The matrix has to be input.
ppv: - a measure of Positive Predictive Value - it is assumed that the input sequences consitute a family, and are all contained in the Swiss-Prot database. PPV measures how certain one can be that a sequence belongs to the family given that it matches the pattern.

For the last three scoring schemes, an input file is needed and option SF appears allowing the user to set his own file name.

Option X:

Exit the menu, and start the pattern discovery process.

Option Q:

Quit Pratt without searching for patterns.

Help Option:

Help is available on-line from the Pratt's menu. Just type help option for help on a specific option from Pratt's menu.

2.4 Controlling the search via command line options.

All parameters whose values can be set using the menu can also be set from the command line. For parameters with numerical value (for example CM (minimum number of sequences to match a pattern) the value of this parameter is set by including -CM <value> in the command line. If this is the only parameter for which you wish to use a non-default value, your command line might look something like
pratt fasta sequences -cm 20
if you wish to look for patterns matching minimum 20 of the sequences in the file sequences (given in FastA format).

For parameters swithchin on or off options, you should include on or off behind the option on the command line. For example if you do not want pattern refinement to be done, your command line might look like
pratt fasta sequences -cm 20 -r off

Nomally if you set some parameters using the command line, Pratt will just start the search using default values for the parameters that you have not set. If you want to see the menu as well, include -menu in the command line.

First run Pratt using a restrictive set of parameter values (for example the default parameters). If it finds a pattern you're happy with, then you're happy. If not, you can run Pratt again using a less restrictive set of parameters, in one of the following ways:

allow for more ambiguous symbols: Increase B.
allow for longer gaps: Increase W.
allow for more flexibilities: Increase N, F, P.
decrease the minimum number of sequences to match a pattern -- decrease M.

Pratt Homepage.