ORFik Overview


Welcome to the ORFik package. ORFik is an R package for analysis of transcript and translation features through manipulation of sequence data and NGS data like Ribo-Seq, RNA-Seq, TCP-Seq and CAGE. It is generalized in the sense that any transcript region can be analysed, as the name hints to, it was made with investigation of ribosomal patterns over Open Reading Frames (ORFs) as it's primary use case.
This vignette will walk you through our detailed package usage with examples.

ORFik currently supports:

  1. Finding Open Reading Frames (very fast) in the genome of interest or on the set of transcripts/sequences.
  2. Hundreds of functions helping your analysis of either: sequence data, RNA-seq data, CAGE data, Ribo-seq data, TCP-seq data or RCP-seq data.
  3. Automatic estimations of RiboSeq footprint shift.
  4. Utilities for metaplots of RiboSeq coverage over gene START and STOP codons allowing to spot the shift.
  5. Shifting functions for the RiboSeq data.
  6. Finding new Transcription Start Sites with the use of CAGE data.
  7. Various measurements of gene identity, more than 30 functions. e.g. FLOSS, coverage, ORFscore, entropy that are recreated based on scientific publications.
  8. Utility functions to extend GenomicRanges for faster grouping, splitting, filtering etc.
  9. Several standardized plots for coverage and metacoverage of NGS data, including smart grouping functions for easier prototyping.
  10. Automatic download of genome annotation from any species supported by ensembl.
  11. Automatic download and renaming of files from SRA, ERA and DRA.
  12. Trimming data using fastp and alignment using STAR.
  13. Simplifying working with massive amounts of datasets using the ORFik experiment class.

The first 9 points on this list is described here, for more advanced use (point 10-13) check out the other vignettes.

Finding Open Reading Frames

In molecular genetics, an Open Reading Frame (ORF) is the part of a reading frame that has the ability to be translated. Although not every ORF has the potential to be translated or to be functional, to find novel genes we must first be able to identify potential ORFs.

To find all Open Reading Frames (ORFs) and possibly map them to genomic coordinates,ORFik gives you three main functions:

Example of finding ORFs in on 5' UTR of hg19

Load libraries we need for examples

library(ORFik)                        # This package
library(GenomicFeatures)              # For basic transcript operations
library(data.table)                   # For fast table operations
library(BSgenome.Hsapiens.UCSC.hg19)  # Human genome

After loading libraries, load human genome from GenomicFeatures.

txdbFile <- system.file("extdata", "hg19_knownGene_sample.sqlite", 
                        package = "GenomicFeatures")

We load gtf file as txdb (transcript database). We will then extract the 5' leaders to find all upstream open reading frames.

txdb <- loadTxdb(txdbFile)
fiveUTRs <- loadRegion(txdb, "leaders")

As we can see we have extracted 5' UTRs for hg19 annotations. Now we can load BSgenome version of human genome (hg19).

Either import fasta or BSgenome file to get sequences.

# Extract sequences of fiveUTRs.
tx_seqs <- extractTranscriptSeqs(BSgenome.Hsapiens.UCSC.hg19::Hsapiens, 

Find all ORFs on those transcripts and get their genomic coordinates.

fiveUTR_ORFs <- findMapORFs(fiveUTRs, tx_seqs, groupByTx = FALSE)

In the example above, you can see that fiveUTR_ORFs are grouped by ORF. That means each group in the GRangesList is 1 ORF, that can have multiple exons. To get the transcript the ORF came from, do this:

txNames(fiveUTR_ORFs[1:2]) # <- Which transcript

You see that both ORFs are from transcript "uc010ogz.1"

Meta-column names contains name of the transcript and identifier of the ORF separated by "_". When a ORF is separated into two exons you see it twice, as the first ORF with name "uc010ogz.1_1". The first ORF will always be the one most upstream for + strand, and the least upstream for - strand.

names(fiveUTR_ORFs[1:2]) # <- Which ORF

Saving ORFs to disc

We recommend two options for storing ORF ranges:

  1. If you want to reuse only in R: Save as R object
saveRDS(fiveUTR_ORFs[1:2], "save/path/uorfs.rds")
  1. If you want to use in IGV, UCSC genome browser etc: Save as bed12 format, that is a bed format with 1 row per ORF, that contains splicing information and even possible color coding for visualizing groups of ORFs:
export.bed12(fiveUTR_ORFs[1:2], "save/path/uorfs.bed12")

Getting sequences from ORFs

Now lets see how easy it is to get fasta sequences from the ranges.

Getting DNA fasta sequences of ORFs

Lets start with the case of getting the DNA sequences.

orf_seqs <- extractTranscriptSeqs(BSgenome.Hsapiens.UCSC.hg19::Hsapiens,

You can see ORF 1 named (uc010ogz.1_1) has a CTG start codon, a TAG stop codon and 159/3 = 53 codons.

To save as .fasta do

writeXStringSet(orf_seqs, filepath = "uorfs.fasta")

Amino acid sequences of ORFs

Now lets do the case of getting the amino acid sequences. We start with the DNA sequences already done from previous step.

To translate to amino acids, following the standard genetic code, do:

orf_aa_seq <- Biostrings::translate(orf_seqs)

save amino acid sequences as .fasta do

writeXStringSet(orf_aa_seq, filepath = "uorfs_AA.fasta")

We will now look on ORFik functions to get startcodons and stopcodon etc.

New GRanges and GRangesList utilities for ORFs

We will now go through utilities to group, subset and filter on interesting regions of ORFs in transcripts.

Grouping ORFs

There are 2 main ways of grouping ORFs. - Group by ORF - Group by transcript

To do this more easily, you can use the function groupGRangesBy.

  1. Grouped by transcript: We use the names() to group,
unlisted_ranges <- unlistGrl(fiveUTR_ORFs)
test_ranges <- groupGRangesBy(unlisted_ranges) # <- defualt is tx grouping by names()

All orfs within a transcript grouped together as one group, the names column seperates the orfs.

  1. Grouped by ORF: we use the orfs meta column called ($names) to group, it is made by ORFik.
unlisted_ranges <- unlistGrl(fiveUTR_ORFs)
test_ranges <- groupGRangesBy(unlisted_ranges, unlisted_ranges$names)

Here you see each group is one ORF only.

Filtering example

Lets say you found some ORFs, and you want to filter out some of them. ORFik provides several functions for filtering. A problem with the original GenomicRanges container is that filtering on GRanges objects are much easier than on a GRangesList object, ORFik tries to fix this.

In this example we will filter out all ORFs as following:

  1. First group GRangesList by ORFs
  2. width < 60
  3. number of exons < 2
  4. strand is negative

Lets use the fiveUTR_ORFs from previous example:

  1. Group by ORFs, if ORFs are grouped by transcripts it would make no sense.
  unlisted_ranges <- unlistGrl(fiveUTR_ORFs)
  ORFs <- groupGRangesBy(unlisted_ranges, unlisted_ranges$names)
  length(ORFs) # length means how many ORFs left in set
  1. Remove widths < 60
  ORFs <- ORFs[widthPerGroup(ORFs) >= 60]
  1. Keep only ORFs with at least 2 exons
  ORFs <- ORFs[numExonsPerGroup(ORFs) > 1]
  1. Keep only positive ORFs
  ORFs <- ORFs[strandPerGroup(ORFs) == "+"]
  # all remaining ORFs where on positive strand, so no change

ORF interest regions

Specific part of the ORF are usually of interest, as start and stop codons. Here we run an example to show what ORFik can do for you.

  1. Find the start and stop sites as GRanges
startSites(fiveUTR_ORFs, asGR = TRUE, keep.names = TRUE, is.sorted = TRUE)
stopSites(fiveUTR_ORFs, asGR = TRUE, keep.names = TRUE, is.sorted = TRUE)
  1. Lets find the start and stop codons. This takes care of potential 1 base exons etc.
starts <- startCodons(fiveUTR_ORFs, is.sorted = TRUE)
stops <- stopCodons(fiveUTR_ORFs, is.sorted = TRUE)
  1. Lets get the bases of the start and stop codons from the fasta file It's very important to check that ORFs are sorted here,
txSeqsFromFa(starts, BSgenome.Hsapiens.UCSC.hg19::Hsapiens, is.sorted = TRUE)
"Stop codons"
txSeqsFromFa(stops, BSgenome.Hsapiens.UCSC.hg19::Hsapiens, is.sorted = TRUE)

Many more operations are also supported for manipulation of ORFs

When to use which ORF-finding function

ORFik supports multiple ORF finding functions. Here we describe their specific use cases:

Which function you will use depend on which organism the data is from, and specific parameters, like circular or non circular genomes, will you use a transcriptome etc.

There are 4 standard ways of finding ORFs:

  1. You have some fasta file of the genome only. (For prokaryotes/circular genomes)
  2. You have some fasta file of the genome and a spliced transcriptome annotation. (For eukaryotes with splicing)
  3. You have a fasta file of transcripts (eukaryotes or prokaryotes)
  4. You have a vector of transcripts preloaded in R.

Let's start with the simplest case; a vector of preloaded transcripts.

Lets say you have some transcripts and want to find all ORFs on them. findORFs() will give you only 5' to 3' direction, so if you want both directions, you can do (for strands in both direction):

  # strand with ORFs in both directions
  ######################>######################< (< > is direction of ORF)

  # positive strands
  pos <- findORFs(seqs, startCodon = "ATG", minimumLength = 0)
  # negative strands
  neg <- findORFs(reverseComplement(seqs),
                  startCodon = "ATG", minimumLength = 0)

Merge into a GRanges object, since we want strand information

  pos <- GRanges(pos, strand = "+")
  neg <- GRanges(neg, strand = "-")
  # as GRanges
  res <- c(pos, neg)
  # or merge together and make GRangesList
  res <- split(res, seq.int(1, length(pos) + length(neg)))

Remember that these results are in transcript coordinates, sometimes you need to convert them to Genomic coordinates.

Finding ORFs in spliced transcripts

If you have a genome and a spliced transcriptome annotation, you must use findMapORFs(). It takes care of the potential problem from the last example, that we really want our result in genomic coordinates in the end.

Prokaryote/Circular Genomes and fasta transcriptomes.

Use findORFsFasta(is.circular = TRUE). Note that findORFsFasta automaticly finds (-) strand too, because that is normally used for genomes.

Filter on strand

If you have fasta transcriptomes, you dont want the (-) strand. Since all transcripts are in the direction in the fasta file. If you get both (+/-) strand and only want (+) ORFs, do:

  res[strandBool(res)] # Keep only + stranded ORFs

See individual functions for more examples.

CageSeq data for 5' UTR re-annotation

In the previous example we used the reference annotation of the 5' UTRs from Hg19.

Here we will use CAGE data to set new Transcription Start Sites (TSS) and re-annotate 5' UTRs. This is useful to improve tissue specific transcripts. Since most eukaryotes usually have variance in TSS definitions.

# path to example CageSeq data from hg19 heart sample
cageData <- system.file("extdata", "cage-seq-heart.bed.bgz", 
                        package = "ORFik")
# get new Transcription Start Sites using CageSeq dataset
newFiveUTRs <- reassignTSSbyCage(fiveUTRs, cageData)

You will now normally see a portion of the transcription start sites have changed. Depending on the species, regular annotations might be incomplete or not specific enough for your purposes.

RiboSeq footprints automatic shift detection and shifting

In RiboSeq data ribosomal footprints are restricted to their p-site positions and shifted with respect to the shifts visible over the start and stop codons. ORFik has multiple functions for processing of RiboSeq data. We will go through an example processing of RiboSeq data below.

Example raw RiboSeq footprints (unshifted):

# Find path to a bam file
bam_file <- system.file("extdata", "ribo-seq.bam", package = "ORFik")
footprints <- readBam(bam_file)

What footprint lengths are present in our data:


Lets look at how the reads distribute around the CDS per read length: For that we need to prepare the transcriptome annotation.

gtf_file <- system.file("extdata", "annotations.gtf", package = "ORFik")
txdb <- loadTxdb(gtf_file)
tx <- exonsBy(txdb, by = "tx", use.names = TRUE)
cds <- cdsBy(txdb, by = "tx", use.names = TRUE)
trailers <- threeUTRsByTranscript(txdb, use.names = TRUE)

Note in ORFik you can load all transcript annotation in one line (this is same as above):

loadRegions(gtf_file, parts = c("tx", "cds", "trailers"))

A note here is that "tx" are all transcripts, if you write "mrna" you will only get subset of tx that has a defined cds.

Restrict footprints to their 5' starts (after shifting it will be a p-site).

footprintsGR <- convertToOneBasedRanges(footprints, addSizeColumn = TRUE)

The function convertToOneBasedRanges gives you a size column, that contains read length information. You can also choose to use the score column for read information. But size has priority over score for deciding what column defines read lengths.

In the figure below we see why we need to p-shift, see that per length the start of the read are in different positions relative to the CDS start site. The reads create a ladder going downwards, left to right. (see the blue steps)

  hitMap <- windowPerReadLength(cds, tx,  footprintsGR, pShifted = FALSE)
  coverageHeatMap(hitMap, scoring = "transcriptNormalized")

If you only want to know how to run the function and no details, skip down to after the 2 coming bar plots.

For the sake of this example we will focus only on most abundant length of 29.

footprints <- footprints[readWidths(footprints) == 29]
footprintsGR <- footprintsGR[readWidths(footprintsGR) == 29]

Filter the cds annotation to only those that have some minimum trailer and leader lengths, as well as cds. Then get start and stop codons with extra window of 30bp around them.

txNames <- filterTranscripts(txdb) # <- get only transcripts that pass filter
tx <- tx[txNames]; cds <- cds[txNames]; trailers <- trailers[txNames];
windowsStart <- startRegion(cds, tx, TRUE, upstream = 30, 29)
windowsStop <- startRegion(trailers, tx, TRUE, upstream = 30, 29)

Calculate meta-coverage over start and stop windowed regions.

hitMapStart <- metaWindow(footprintsGR, windowsStart, withFrames = TRUE)
hitMapStop <- metaWindow(footprintsGR, windowsStop, withFrames = TRUE)

Plot start/stop windows for length 29.

  pSitePlot(hitMapStop, region = "stop")

From these shifts ORFik uses a fourier transform to detect signal change needed to scale all read lengths of Ribo-seq to the start of the meta-cds.

We can also use automatic detection of RiboSeq shifts using the code below. As we can see reasonable conclusion from the plots would be to shift length 29 by 12, it is in agreement with the automatic detection of the offsets.

shifts <- detectRibosomeShifts(footprints, txdb, stop = TRUE)

Fortunately ORFik has a function that can be used to shift footprints using desired shifts. See documentation for more details.

shiftedFootprints <- shiftFootprints(footprints, shifts)

Gene identity functions for ORFs or genes

ORFik contains functions of gene identity that can be used to predict which ORFs are potentially coding and functional.

There are 2 main categories:

Some important read features are:

All of the features are implemented based on scientific article published in peer reviewed journal. ORFik supports seamless calculation of all available features. See example below.

Find ORFs:

fiveUTRs <- fiveUTRs[1:10]
faFile <- BSgenome.Hsapiens.UCSC.hg19::Hsapiens
tx_seqs <- extractTranscriptSeqs(faFile, fiveUTRs)

fiveUTR_ORFs <- findMapORFs(fiveUTRs, tx_seqs, groupByTx = FALSE)

Make some toy ribo seq and rna seq data:

starts <- unlist(ORFik:::firstExonPerGroup(fiveUTR_ORFs), use.names = FALSE)
RFP <- promoters(starts, upstream = 0, downstream = 1)
score(RFP) <- rep(29, length(RFP)) # the original read widths
# set RNA-seq seq to duplicate transcripts
RNA <- unlist(exonsBy(txdb, by = "tx", use.names = TRUE), use.names = TRUE)

Find features of sequence and library data

# transcript database
txdb <- loadTxdb(txdbFile)
dt <- computeFeatures(fiveUTR_ORFs[1:4], RFP, RNA, txdb, faFile, 
                      sequenceFeatures = TRUE)

You will now get a data.table with one column per score, the columns are named after the different scores, you can now go further with prediction, or making plots.

Calculating Kozak sequence score for ORFs

Instead of getting all features, we can also extract single features.

To understand how strong the binding affinitity of an ORF promoter region might be, we can use kozak sequence score. The kozak functions supports several species. In the first example we use human kozak sequence, then we make a self defined kozak sequence.

In this example we will find kozak score of cds'

cds <- cdsBy(txdb, by = "tx", use.names = TRUE)[1:10]
tx <- exonsBy(txdb, by = "tx", use.names = TRUE)[names(cds)]
faFile <- BSgenome.Hsapiens.UCSC.hg19::Hsapiens

kozakSequenceScore(cds, tx, faFile, species = "human")

A few species are pre supported, if not, make your own input pfm.

Here is an example where the human pfm is sent in again, even though it is already supported:

pfm <- t(matrix(as.integer(c(29,26,28,26,22,35,62,39,28,24,27,17,
                ncol = 4))

kozakSequenceScore(cds, tx, faFile, species = pfm)

As an example of the many plots you can make with ORFik, let's make a scoring of Ribo-seq by kozak sequence.

seqs <- startRegionString(cds, tx, faFile, upstream = 5, downstream = 5)
rate <- fpkm(cds, RFP)
ORFik:::kozakHeatmap(seqs, rate)

It will be a black boundary box around the strongest nucleotide per position (what base at what position gives highest ribo-seq fpkm for the cds). See at the start codon (position +1 to +3) you have A, T, G. As known from the literature many C's before start codon and a G after the start codon. In a real example most of the nucleotides will be used for all positions.

Using ORFik in your package or scripts

The focus of ORFik for development is to be a Swiss army knife for transcriptomics. If you need functions for splicing, getting windows of exons per transcript, periodic windows of exons, speicific parts of exons etc, ORFik can help you with this.

Let's do an example where ORFik shines. Objective: We have three transcripts, we also have a library of Ribo-seq. This library was treated with cyclohexamide, so we know Ribo-seq reads can stack up close to the stop codon of the CDS. Lets say we only want to keep transcripts, where the cds stop region (defined as last 9 bases of cds), has maximum 33% of the reads. To only keep transcripts with a good spread of reads over the CDS. How would you make this filter ?

  # First make some toy example
  cds <- GRanges("chr1", IRanges(c(1, 10, 20, 30, 40, 50, 60, 70, 80),
                                 c(5, 15, 25, 35, 45, 55, 65, 75, 85)),
  names(cds) <- c(rep("tx1", 3), rep("tx2", 3), rep("tx3", 3))
  cds <- groupGRangesBy(cds)
  ribo <- GRanges("chr1", c(1, rep.int(23, 4), 30, 34, 34, 43, 60, 64, 71, 74),
  # We could do a simplification and use the ORFik entropy function
  entropy(cds, ribo) # <- spread of reads

We see that ORF 1, has a low (bad) entropy, but we do not know where the reads are stacked up. So lets make a new filter by using more ORFiks utility functions:

tile <- tile1(cds, FALSE, FALSE) # tile them to 1 based positions
tails <- tails(tile, 9) # get 9 last bases per cds
stopOverlap <- countOverlaps(tails, ribo)
allOverlap <- countOverlaps(cds, ribo)
fractions <- (stopOverlap + 1) / (allOverlap + 1) # pseudocount 1
cdsToRemove <- fractions > 1 / 2 # filter with pseudocounts (1+1)/(3+1) 

We now easily made a stop codon filter for our coding sequences.

Coverage plots made easy with ORFik

In investigation of ORFs or other interest regions, ORFik can help you make some coverage plots from reads of Ribo-seq, RNA-seq, CAGE-seq, TCP-seq etc.

Lets make 3 plots of Ribo-seq focused on CDS regions.

Load data as shown before and pshift the Ribo-seq:

# Get the annotation
txdb <- loadTxdb(gtf_file)
# Ribo-seq
bam_file <- system.file("extdata", "ribo-seq.bam", package = "ORFik")
reads <- readGAlignments(bam_file)
shiftedReads <- shiftFootprints(reads, detectRibosomeShifts(reads, txdb))  

Make meta windows of leaders, cds' and trailers

# Lets take all valid transcripts, with size restrictions:
# leader > 100 bases, cds > 100 bases, trailer > 100 bases
txNames <- filterTranscripts(txdb, 100, 100, 100) # valid transcripts
loadRegions(txdb, parts = c("leaders", "cds", "trailers", "tx"), 
            names.keep = txNames)

# Create meta coverage per part of transcript
leaderCov <- metaWindow(shiftedReads, leaders, scoring = NULL, 
                        feature = "leaders")

cdsCov <- metaWindow(shiftedReads, cds, scoring = NULL, 
                     feature = "cds")

trailerCov <- metaWindow(shiftedReads, trailers, scoring = NULL, 
                         feature = "trailers")

Bind together and plot:

dt <- rbindlist(list(leaderCov, cdsCov, trailerCov))
dt[, `:=` (fraction = "Ribo-seq")] # Set info column
# zscore gives shape, a good starting plot
windowCoveragePlot(dt, scoring = "zscore", title = "Ribo-seq metaplot") 

Z-score is good at showing overall shape. You see from the windows each region; leader, cds and trailer is scaled to 100. NOTE: we can use the function windowPerTranscript to do all of this in one call.

Lets use a median scoring to find median counts per meta window per positions.

windowCoveragePlot(dt, scoring = "median", title = "Ribo-seq metaplot") 

We see a big spike close to start of CDS, called the TIS. The median counts by transcript is close to 50 here. Lets look at the TIS region using the pshifting plot, seperated into the 3 frames.

if (requireNamespace("BSgenome.Hsapiens.UCSC.hg19")) {
  # size 100 window: 50 upstream, 49 downstream of TIS
  windowsStart <- startRegion(cds, tx, TRUE, upstream = 50, 49)
  hitMapStart <- metaWindow(shiftedReads, windowsStart, withFrames = TRUE)
  pSitePlot(hitMapStart, length = "meta coverage")

Since these reads are p-shifted, the maximum number of reads are on the 0 position. We also see a clear pattern in the Ribo-seq.

To see how the different read lengths distribute over the region, we make a heatmap. Where the colors represent the zscore of counts per position.

hitMap <- windowPerReadLength(cds, tx,  shiftedReads)
coverageHeatMap(hitMap, addFracPlot = TRUE)

In the heatmap you can see that read length 30 has the strongest peak on the TIS, while read length 28 has some reads in the leaders (the minus positions).

Multiple data sets in one plot

Often you have multiple data sets you want to compare (like ribo-seq).

ORFik has an extensive syntax for automatic grouping of data sets in plots.

The protocol is: 1. Load all data sets 2. Create a merged coverage data.table 3. Pass it into the plot you want.

Here is an easy example:

if (requireNamespace("BSgenome.Hsapiens.UCSC.hg19")) {
  # Load more files like above (Here I make sampled data from earlier Ribo-seq)
  dt2 <- copy(dt)
  dt2[, `:=` (fraction = "Ribo-seq2")]
  dt2$score <- dt2$score + sample(seq(-40, 40), nrow(dt2), replace = TRUE)

  dtl <- rbindlist(list(dt, dt2))
  windowCoveragePlot(dtl, scoring = "median", title = "Ribo-seq metaplots") 

You see that the fraction column is what seperates the rows. You can have unlimited datasets joined in this way. For more useful examples of multilibrary plotting continue with the vignette called ORFikExperiment (Data management).


Our hope is that by using ORFik, we can simplify your analysis when you focus on ORFs / transcript features and especially in combination with sequence libraries like RNA-seq and Ribo-seq.

If needed, you can move to the more advanced features of ORFik in the next vignettes, Happy coding!

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ORFik documentation built on March 27, 2021, 6 p.m.