Multiple Files (Loops)

A colleague wants to do a comparitive analysis of the GC content (i.e., the percentage of bases in a DNA sequence that are either G or C) on data from the PATRIC bacterial phytogenomic database. They want to know the GC content of all of the bacteria in the database and have started working initially with a handful of archaea. They know a little R and wrote the following code to load a single sequence and caculate it’s GC content.

library(ShortRead)
library(Biostrings)

reads <- readFasta("archaea-dna/a-saccharovorans.fasta")
seq <- sread(reads)
base_freq <- alphabetFrequency(seq)
gc_content <- (base_freq[1, "G"] + base_freq[1, "C"]) / sum(base_freq) * 100

The first two lines load a .fasta file using the ShortRead package in Bioconductor. The second two lines determine the frequency of all of the bases in sequence and then calculate the GC content.

Start by installing Bioconductor with the following code (this may take a while, so be patient):

source("https://bioconductor.org/biocLite.R")
biocLite("ShortRead")
biocLite("Biostrings")

Each fasta file contains one long sequence of DNA for an archaea species. The following code loads one sequence file, where seq is the variable name for the data file:

library(ShortRead)
reads <- readFasta("archaea-dna/a-saccharovorans.fasta")
seq <- sread(reads)

Help out your colleagues by downloading the data and looping over the files to determine the GC content for each file. Once downloaded, either unzip the .zip file manually or use the unzip function.

Use list.files(), with full.names set to true, to generate a list of the names of all the sequence files. Then create a for loop that uses the above code to read in each sequence file and calculate it’s GC content. Store the resulting values in a data frame with one column with file names and a second column with GC contents.