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Codon usage bias is a phenomenon whereby different organisms exhibit distinct preferences for synonymous codons, which are multiple codons that encode the same amino acid. This variation in codon usage patterns is observed across all levels of life, from bacteria to eukaryotes. Codon usage bias is influenced by a variety of factors, including gene expression, GC content, and horizontal gene transfer. Understanding the causes and consequences of codon usage bias is important for a variety of fields, including molecular biology, evolutionary biology, and biotechnology.

cubar can be a helpful tool for researchers who are interested in studying codon usage bias. It provides a variety of functions that can be used to calculate and visualize codon usage bias metrics.

Here, we demonstrate the basic functionalities of cubar by analyzing the coding sequences (CDSs) of brewer’s yeast as an example.

suppressPackageStartupMessages(library(Biostrings))
#> Warning: package 'Biostrings' was built under R version 4.3.3
#> Warning: package 'GenomeInfoDb' was built under R version 4.3.3
suppressPackageStartupMessages(library(data.table))
#> Warning: package 'data.table' was built under R version 4.3.3
library(cubar)
library(ggplot2)

Sequences and the Genetic Code

First, quality control was performed on the provided Yeast CDS sequences to ensure that each sequence had the correct start codon, stop codon, and no internal stop codons. Additionally, the length of each sequence was verified to be a multiple of three. These QC procedures can be adjusted based on the input sequences. For example, if your sequences do not contain 3’ stop codons, you can skip this check by setting check_stop = FALSE.

# example data
yeast_cds
#> DNAStringSet object of length 6600:
#>        width seq                                            names               
#>    [1]   471 ATGAGTTCCCGGTTTGCAAGAA...GATGTGGATATGGATGCGTAA YPL071C
#>    [2]   432 ATGTCTAGATCTGGTGTTGCTG...AGAGGCGCTGGTTCTCATTAA YLL050C
#>    [3]  2160 ATGTCTGGAATGGGTATTGCGA...GAGAGCCTTGCTGGAATATAG YMR172W
#>    [4]   663 ATGTCAGCACCTGCTCAAAACA...GAAGACGATGCTGATTTATAA YOR185C
#>    [5]  2478 ATGGATAACTTCAAAATTTACA...TATCAAAATGGCAGAAAATGA YLL032C
#>    ...   ... ...
#> [6596]  1902 ATGCCAGACAATCTATCATTAC...CACGAAAAGACTTTCATTTAA YBR021W
#> [6597]   138 ATGAGGGTTCTCCATGTTATGC...AAAAAAAAAAAAAAAAGATGA YDR320W-B
#> [6598]   360 ATGTTTATTCTAGCAGAGGTTT...AATGCCGCGCTGGACGATTAA YBR232C
#> [6599]  1704 ATGGCAAGCGAACAGTCCTCAC...TTCCCAAAGAGTTTTAATTGA YDL245C
#> [6600]   906 ATGTTGAATAGTTCAAGAAAAT...TACTCTTTTATCTTCAATTGA YBR024W

# qc
yeast_cds_qc <- check_cds(yeast_cds)
yeast_cds_qc
#> DNAStringSet object of length 6574:
#>        width seq                                            names               
#>    [1]   465 AGTTCCCGGTTTGCAAGAAGTA...ACTGATGTGGATATGGATGCG YPL071C
#>    [2]   426 TCTAGATCTGGTGTTGCTGTTG...AGCAGAGGCGCTGGTTCTCAT YLL050C
#>    [3]  2154 TCTGGAATGGGTATTGCGATTC...CAAGAGAGCCTTGCTGGAATA YMR172W
#>    [4]   657 TCAGCACCTGCTCAAAACAATG...GATGAAGACGATGCTGATTTA YOR185C
#>    [5]  2472 GATAACTTCAAAATTTACAGTA...AAATATCAAAATGGCAGAAAA YLL032C
#>    ...   ... ...
#> [6570]  1896 CCAGACAATCTATCATTACATT...GAACACGAAAAGACTTTCATT YBR021W
#> [6571]   132 AGGGTTCTCCATGTTATGCTTT...ATGAAAAAAAAAAAAAAAAGA YDR320W-B
#> [6572]   354 TTTATTCTAGCAGAGGTTTCGG...TTTAATGCCGCGCTGGACGAT YBR232C
#> [6573]  1698 GCAAGCGAACAGTCCTCACCAG...AAGTTCCCAAAGAGTTTTAAT YDL245C
#> [6574]   900 TTGAATAGTTCAAGAAAATATG...TGGTACTCTTTTATCTTCAAT YBR024W

CDSs sequences can be convert to codon sequences by seq_to_codons or translated to corresponding amino acid sequences with translate from Biostrings.

# convert a CDS to codon sequence
seq_to_codons(yeast_cds_qc[['YDR320W-B']])
#>  [1] "AGG" "GTT" "CTC" "CAT" "GTT" "ATG" "CTT" "TCT" "TTC" "CTA" "AAC" "TCA"
#> [13] "CTT" "CTT" "TTC" "CTC" "CCT" "ATC" "TGC" "TTT" "TGT" "TTA" "TTA" "CAG"
#> [25] "TTG" "AAG" "GCT" "ACT" "TGT" "GCC" "GTT" "CGT" "GTG" "AAA" "AAA" "TAC"
#> [37] "TCG" "ATG" "AAA" "AAA" "AAA" "AAA" "AAA" "AGA"

# convert a CDS to amino acid sequence
Biostrings::translate(yeast_cds_qc[['YDR320W-B']])
#> 44-letter AAString object
#> seq: RVLHVMLSFLNSLLFLPICFCLLQLKATCAVRVKKYSMKKKKKR

Many codon usage metrics depend on codon frequencies, which can be calculated easily by the function count_codons.

# get codon frequency
yeast_cf <- count_codons(yeast_cds_qc)

In the resulting matrix, each row represents a gene, and each column represents a codon. The values in the matrix represent the frequency of each codon in the corresponding gene.

yeast_cf[1:3, 1:3]
#>         AAA AAC AAG
#> YPL071C  10   4   5
#> YLL050C   6   3   5
#> YMR172W  16  37  25

To interact with the genetic code, cubar provided a helpful function to convert genetic code in Biostrings to a handy table and an option to visualize possible codon-anticodon pairing.

# get codon table for the standard genetic code
ctab <- get_codon_table(gcid = '1')

# plot possible codon and anticodon pairings
plot_ca_pairing(ctab)

Alternatively, user can create a custom genetic code table by providing a mapping between amino acids and codons.

# example of a custom mapping
head(aa2codon)
#>   amino_acid codon
#> 1          *   TAA
#> 2          *   TAG
#> 3          *   TGA
#> 4        Ala   GCT
#> 5        Ala   GCC
#> 6        Ala   GCA

# create a custom codon table
custom_ctab <- create_codon_table(aa2codon)
head(custom_ctab)
#>    aa_code amino_acid  codon subfam
#>     <char>     <char> <char> <char>
#> 1:       *          *    TAA   *_TA
#> 2:       *          *    TAG   *_TA
#> 3:       *          *    TGA   *_TG
#> 4:       A        Ala    GCT Ala_GC
#> 5:       A        Ala    GCC Ala_GC
#> 6:       A        Ala    GCA Ala_GC

Codon usage indices

Most indices can be calculate with get_* series functions and the return value is usually a vector with value names identical to the names of sequences. Here we demonstrate how to calculate various indices with the above yeast CDS data.

Effective Number of Codons (ENC)

# get enc
enc <- get_enc(yeast_cf)
head(enc)
#>  YPL071C  YLL050C  YMR172W  YOR185C  YLL032C  YBR225W 
#> 53.00343 45.06356 56.01914 50.84984 53.29440 53.82957

plot_dist <- function(x, xlab = 'values'){
    x <- stack(x)
    ggplot(x, aes(x = values)) +
        geom_histogram(bins = 40, fill = '#88CCEE') +
        labs(x = xlab, y = 'Number of genes') +
        theme_classic(base_size = 12) +
        theme(axis.text = element_text(color = 'black'))
}

plot_dist(enc, 'ENC')

Fraction of optimal codons (Fop)

# get fop
fop <- get_fop(yeast_cf)
plot_dist(fop, 'Fop')

cubar provides a method to determine the optimal (or “preferred”) codon for each codon subfamily based on regression of codon usage against scores for genes. Preferred codons are more likely to be used in genes with high scores. Consequently, preferred codons will have positive coefficients in the regression analysis. Users can provide a vector of their own gene scores, for example, log1p-transformed gene expression levels (RPKM or TPM). It worthy noting that the order of gene scores should match the order of genes in the codon frequency matrix. Otherwise, the results will be meaningless. If gene scores were not provided, cubar will use the opposite of ENC by default (so that genes with stronger codon usage bias have larger scores).

To view the optimal codons, you can manually run the est_optimal_codons function.

optimal_codons <- est_optimal_codons(yeast_cf, codon_table = ctab)
head(optimal_codons[optimal == TRUE])
#>    aa_code amino_acid  codon subfam       coef        pvalue        qvalue
#>     <char>     <char> <char> <char>      <num>         <num>         <num>
#> 1:       A        Ala    GCT Ala_GC 0.08702058  0.000000e+00  0.000000e+00
#> 2:       A        Ala    GCC Ala_GC 0.01876569  2.164990e-40  2.401170e-40
#> 3:       R        Arg    AGA Arg_AG 0.13023392  0.000000e+00  0.000000e+00
#> 4:       R        Arg    CGT Arg_CG 0.21009663  0.000000e+00  0.000000e+00
#> 5:       N        Asn    AAC Asn_AA 0.05752254 1.600668e-292 2.871787e-292
#> 6:       D        Asp    GAC Asp_GA 0.01846525  4.298281e-36  4.599915e-36
#>    optimal
#>     <lgcl>
#> 1:    TRUE
#> 2:    TRUE
#> 3:    TRUE
#> 4:    TRUE
#> 5:    TRUE
#> 6:    TRUE

Codon Adaptation Index (CAI)

# estimate RSCU of highly expressed genes
yeast_exp <- as.data.table(yeast_exp)
yeast_exp <- yeast_exp[gene_id %in% rownames(yeast_cf), ]
yeast_heg <- head(yeast_exp[order(-fpkm), ], n = 500)
rscu_heg <- est_rscu(yeast_cf[yeast_heg$gene_id, ], codon_table = ctab)
head(rscu_heg) # RSCU values are shown in the column `rscu`
#>    aa_code amino_acid  codon subfam   cts      prop     w_cai      rscu
#>     <char>     <char> <char> <char> <num>     <num>     <num>     <num>
#> 1:       F        Phe    TTT Phe_TT  2710 0.4013918 0.6705417 0.8027835
#> 2:       F        Phe    TTC Phe_TT  4042 0.5986082 1.0000000 1.1972165
#> 3:       L        Leu    TTA Leu_TT  3231 0.3234264 0.4780358 0.6468528
#> 4:       L        Leu    TTG Leu_TT  6760 0.6765736 1.0000000 1.3531472
#> 5:       S        Ser    TCT Ser_TC  4646 0.4897249 1.0000000 1.9588998
#> 6:       S        Ser    TCC Ser_TC  2892 0.3048793 0.6225522 1.2195173

# calculate CAI of all genes
# note: CAI values are usually calculated based RSCU of highly expressed genes.
cai <- get_cai(yeast_cf, rscu = rscu_heg)
plot_dist(cai, xlab = 'CAI')

tRNA Adaptation Index (tAI)

# get tRNA gene copy number from GtRNADB
trna_gcn <- table(data.table::tstrsplit(sub(' .*', '', names(yeast_trna)), '-')[[3]])
trna_gcn <- trna_gcn[names(trna_gcn) != 'NNN'] # copy of each anticodon

# calculate tRNA weight for each codon
trna_w <- est_trna_weight(trna_level = trna_gcn, codon_table = ctab)

# get tAI
tai <- get_tai(yeast_cf, trna_w = trna_w)
plot_dist(tai, 'tAI')

Note that cubar has an internal copy of yeast_trna. You can also download mature tRNA sequences from the GtRNADB website (if you are lucky to have good internet connection) and read them into R using the following code:

# path_gtrnadb <- 'http://gtrnadb.ucsc.edu/genomes/eukaryota/Scere3/sacCer3-mature-tRNAs.fa'
# yeast_trna <- Biostrings::readRNAStringSet(path_gtrnadb)

Correlation between indices

pairs(cbind(CAI = cai, ENC = enc, Fop = fop, TAI = tai),
      cex = 0.5, col = alpha('black', 0.2))

Utilities

Test of differential usage

cubar provides a function to test for differential codon usage between two sets of sequences. The function codon_diff calculates the odds ratio and p-value for each codon, comparing the usage in the two sets of sequences. The function returns a data table with the results of the global, family, and subfamily tests.

Here, we compare the codon usage of lowly expressed genes with highly expressed genes in yeast.

# get lowly expressed genes
yeast_leg <- head(yeast_exp[order(fpkm), ], n = 500)
yeast_leg <- yeast_leg[gene_id %in% rownames(yeast_cf), ]

# differetial usage test
du_test <- codon_diff(yeast_cds_qc[yeast_heg$gene_id], yeast_cds_qc[yeast_leg$gene_id])
du_test <- du_test[amino_acid != '*', ]

The results of the differential usage test can be visualized using a bar plot of the odds ratios for each codon. Codons with odds ratios greater than 1 are used more frequently in the highly expressed genes, while codons with odds ratios less than 1 are used more frequently in the lowly expressed genes.

du_test$codon <- factor(du_test$codon, levels = du_test[order(-global_or), codon])

ggplot(du_test, aes(x = codon, y = log2(global_or))) +
    geom_col() +
    labs(x = NULL, y = 'log2(OR)') +
    theme_classic(base_size = 12) +
    theme(axis.text = element_text(color = 'black'),
          axis.text.x = element_text(angle = 90, vjust = 0.5, hjust = 1))

cubar also tests for differences in codon usage at the family and subfamily levels.

du_test2 <- du_test[!amino_acid %in% c('Met', 'Trp'), ]
du_test2$codon <- factor(du_test2$codon, levels = du_test2[order(-fam_or), codon])

ggplot(du_test2, aes(x = codon, y = log2(fam_or))) +
    geom_col() +
    labs(x = NULL, y = 'log2(OR)') +
    facet_grid(cols = vars(amino_acid), space = 'free', scales = 'free', drop = TRUE) +
    theme_light() +
    theme(axis.text.x = element_text(angle = 90, vjust = 0.5, hjust = 1))

Codon usage optimization

cubar provides a function to optimize codon usage for heterologous expression. Here is an example of optimizing the codon usage of the yeast gene YFR025C (HIS2) based on the optimal codons calculated earlier.

# optimize codon usage to the optimal codon of each amino acid
opc_aa <- est_optimal_codons(yeast_cf, codon_table = ctab, level = 'amino_acid')
seq_optimized <- codon_optimize(yeast_cds_qc[['YFR025C']], optimal_codons, level = 'amino_acid')

# CAI before and after optimization
plot_dist(cai, 'CAI') + 
    geom_vline(xintercept = cai['YFR025C'], linetype = 'dashed', color = 'red') +  # before
    geom_vline(xintercept = get_cai(count_codons(seq_optimized), rscu_heg),
               linetype = 'dashed', color = 'black')  # after

Sliding-window analysis

cubar provides a function to perform a sliding-window analysis of codon usage bias. This analysis can be useful for identifying regions of a gene that exhibit distinct codon usage patterns. Here, we demonstrate how to perform a sliding-window analysis on YLR106C, one of the longest yeast genes.

swa <- slide_apply(yeast_cds_qc[['YHR099W']], .f = get_cai,
                   step = 30, before = 20, after = 20, rscu = rscu_heg)

# plot the results
slide_plot(swa, 'CAI')

FAQ

What do families and subfamilies mean in cubar? > A codon family is the set of codons encoding the same amino acid. For a large codon family that has more than four synonymous codons, cubar will break it into two subfamilies depending on the first two nucleotides of codons. For example, leucine is encoded by six codons in the standard genetic code. cubar will break the six codons into two subfamilies: Leu_UU for UUA and UUG; Leu_CU for CUU, CUC, CUA, and CUG. Unless otherwise stated, codon weights for most indices are calculated at the subfamily level. However, there are options to estimate optimal codons and perform codon optimization at the family level to suit different user needs.