The aim of the analysis is to determine the positions or regions of aligned sequences where chosen physico–chemical or conformational feature variations depend on each other. Such dependence could indicate possible importance of these regions for functioning of the sequences and can point to possible molecular mechanisms of functioning of these regulatory sequences.

The method is based on traditional approach designed for pairwise correlation analysis of amino acid sequence samples [Neher E., 1994]. This method was modified for detailed analysis of compensatory substitutions of residues in positions of sequences referring to different protein families [Afonnikov D.A. et al., 2000].

For analysing aligned sample set of DNA site nucleotide sequences, we use conformational DNA properties calculated for nucleotide pairs [Ponomarenko J.V. et al., 1999]. We consider a set of N aligned sequences with the length L. In the process of analysis, each sequence is being re-coded into the sequence of nucleotides with the length L-1. Each dinucleotide is corresponded to a particular value of physico-chemical or conformational property f. Thus, we arrive at numerical matrix NxL-1. An element [l,m] of this matrix corresponds to the value of property f of a dinucleotide beginning at position m in the sequence l.

As the measure of dependency between the values of properties f in positions of the sample sequences, we take the value of correlation coefficient as follows:

, (1)

where x_i is a property of a nucleotide starting at i– th position,

N is a number of sequences in a set,

d_ii is a sample variance of a property of a dinucleotide starting at i-th position.

Significant deviation of correlation coefficient from zero (that is, when it exceeds by modulo some limiting value r_limit.) means that there exists a statistical dependency for conformational property values at a pair of positions i, j. To determine r_limit. , we use the value

, (2)

which is distributed in accordance with the Student criterion with m = N-2 degrees of freedom [Anderson T.W., 1958].

For analysis of nucleotide sequences, the problems, related to determining of statistical dependencies in a set given, do not appear due to evolutionary interrelationships between the sequences [Afonnikov D.A. et al, 2000]. This fact simplifies an application of the method.

To reveal the blocks with significant prevalence of correlating pairs in the matrix it undergoes the procedure of clustering. The size of the blocks were chosen so that the number of pairs forming the square block should be essentially less than their complete number of pairs in the whole matrix. In order to determine statistical reliability of prevalence of correlating pairs in this block, we have used an approximation of binomial distribution. In this case, the probability p of event that the number of significant pairs in this block should exceed the observed value m equals to

,

where q is a probability of significant correlation for the whole matrix, n is the number of positions in a window.

Thus, this block was considered as significant if p < 1%, that is, this corresponds to significance level equalling to 99%.

How to use

Enter the alignment to be analysed (in FASTA format) into the ‘Sequence alignment’ text-box

(the alignment can be extracted out of any DNA sequence alignment database like Samples. The sequence number should be at least 10. Note that accuracy of the method increases with sequence number. The sequences should be of equal length that should not exceed 130 bp. Gaps are forbidden, so positions of alignment that contain gaps will be eliminated from analysis).

2. Choose the conformational or physico-chemical property of DNA molecule

that you want to analyse in the ‘Dinucleotide property’drop-down menu:

3.Choose the form of the output data display and significance level in the ‘Output parameters setup’ menus:

4. Type the number in the range 1-20 in the text-box ‘Window size’ to choose the size of window for clusterisation process:

Numbers between 3 and 10 recommended

5. Click ‘Submit button’ to display the results automatically.