bio3d (version 2.4-4)

entropy: Shannon Entropy Score

Description

Calculate the sequence entropy score for every position in an alignment.

Usage

entropy(alignment)

Value

Returns a list with five components:

H

standard entropy score for a 22-letter alphabet.

H.10

entropy score for a 10-letter alphabet (see below).

H.norm

normalized entropy score (for 22-letter alphabet), so that conserved (low entropy) columns (or positions) score 1, and diverse (high entropy) columns score 0.

H.10.norm

normalized entropy score (for 10-letter alphabet), so that conserved (low entropy) columns score 1 and diverse (high entropy) columns score 0.

freq

residue frequency matrix containing percent occurrence values for each residue type.

Arguments

alignment

sequence alignment returned from read.fasta or an alignment character matrix.

Author

Barry Grant

Details

Shannon's information theoretic entropy (Shannon, 1948) is an often-used measure of residue diversity and hence residue conservation.

References

Grant, B.J. et al. (2006) Bioinformatics 22, 2695--2696.

Shannon (1948) The System Technical J. 27, 379--422.

Mirny and Shakhnovich (1999) J. Mol. Biol. 291, 177--196.

See Also

consensus, read.fasta

Examples

# Read HIV protease alignment 
aln <- read.fasta(system.file("examples/hivp_xray.fa",package="bio3d"))

# Entropy and consensus
h   <- entropy(aln)
con <- consensus(aln)

names(h$H)=con$seq
print(h$H)

# Entropy for sub-alignment (positions 1 to 20) 
h.sub <- entropy(aln$ali[,1:20])

# Plot entropy and residue frequencies (excluding positions >=60 percent gaps)
H <- h$H.norm
H[ apply(h$freq[21:22,],2,sum)>=0.6 ] = 0

col <- mono.colors(32)
aa  <- rev(rownames(h$freq))
oldpar <- par(no.readonly=TRUE)
layout(matrix(c(1,2),2,1,byrow = TRUE), widths = 7, 
       heights = c(2, 8), respect = FALSE)

# Plot 1: entropy
par(mar = c(0, 4, 2, 2))
barplot(H, border="white", ylab = "Entropy",
        space=0, xlim=c(3.7, 97.3),yaxt="n" )
axis(side=2, at=c(0.2,0.4, 0.6, 0.8))
axis(side=3, at=(seq(0,length(con$seq),by=5)-0.5),
     labels=seq(0,length(con$seq),by=5))
box()

# Plot2: residue frequencies 
par(mar = c(5, 4, 0, 2))
image(x=1:ncol(con$freq),
      y=1:nrow(con$freq),
      z=as.matrix(rev(as.data.frame(t(con$freq)))),
      col=col, yaxt="n", xaxt="n",
      xlab="Alignment Position", ylab="Residue Type")
axis(side=1, at=seq(0,length(con$seq),by=5))
axis(side=2, at=c(1:22), labels=aa)
axis(side=3, at=c(1:length(con$seq)), labels =con$seq)
axis(side=4, at=c(1:22), labels=aa)
grid(length(con$seq), length(aa))
box()

for(i in 1:length(con$seq)) {
  text(i, which(aa==con$seq[i]),con$seq[i],col="white")
}
abline(h=c(3.5, 4.5, 5.5, 3.5, 7.5, 9.5,
         12.5, 14.5, 16.5, 19.5), col="gray")

par(oldpar)