Codon usage determines translation rate in Escherichia coli

We wish to determine whether differences in translation rate are correlated with differences in codon usage or with differences in mRNA secondary structure. We therefore inserted a small DNA fragment in the lacZ gene either directly or flanked by a few frame-shifting bases, leaving the reading frame of the lacZ gene unchanged. The fragment was chosen to have "infrequent" codons in one reading frame and "common" codons in the other. The insert in these constructs does not seem to give mRNAs that are able to form extensive secondary structures. The translation time for these modified lacZ mRNAs was measured with a reproducibility better than plus or minus one second. We found that the mRNA with infrequent codons inserted has an approximately three-seconds longer translation time than the one with common codons. In another set of experiments we constructed two almost identical lacZ genes in which the lacZ mRNAs have the potential to generate stem structures with stabilities of about -75 kcal/mol. In this way we could investigate the influence of mRNA structure on translation rate. This type of modified gene was generated in two reading frames with either common or infrequent codons similar to our first experiments. We find that the yield of protein from these mRNAs is reduced, probably due to the action in vivo of an RNase. Nevertheless, the data do not indicate that there is any effect of mRNA secondary structure on translation rate. In contrast, our data persuade us that there is a difference in translation rate between infrequent codons and common codons that is of the order of sixfold.     

Codon usage changes and sequence dissimilarity between human and rat

Summary This paper reports on the relationship between the number of silent differences and the codon usage changes in the lineages leading to human and rat. Examination of 102 pairs of homologous genes gives rise to four main conclusions: (1) We have previously demonstrated the existence of a codon usage change (called the minor shift) between human and rat; this was confirmed here with a larger sample. For genes with extreme C+G frequencies, the C+G level in the third codon position is less extreme in rat than in human. (2) Protein similarity and percentage of positive differences are the two main factors that discriminate homologous genes when characterized by differences between rat and human. By definition, positive differences result from silent changes between A or T and C or G with a direction implying a C+G content variation in the same direction as the overall gene variation. (3) For genes showing both codon usage change and low protein similarity, a majority of amino acid replacements contributes to C+G level variation in positions I and II in the same direction as the variation in position III. This is thus a new example of protein evolution due to constraints acting at the DNA level. (4) In heavy isochores (high C+G content) no direct correlation exists between codon usage change (measured by the dissymmetry of differences) and silent dissimilarity. In light isochores the opposite situation is observed: modification of codon usage is associated with a high synonymous dissimilarity. This result shows that, in some cases, modification of constraints acting at the DNA level could accelerate divergence between genomes.     

Codon usage in bacteria: correlation with gene expressivity

The nucleic acid sequence bank now contains over 600 protein coding genes of which 107 are from prokaryotic organisms. Codon frequencies in each new prokaryotic gene are given. Analysis of genetic code usage in the 83 sequenced genes of the Escherichia coli genome (chromosome, transposons and plasmids) is presented, taking into account new data on gene expressivity and regulation as well as iso-tRNA specificity and cellular concentration. The codon composition of each gene is summarized using two indexes: one is based on the differential usage of iso-tRNA species during gene translation, the other on choice between Cytosine and Uracil for third base. A strong relationship between codon composition and mRNA expressivity is confirmed, even for genes transcribed in the same operon. The influence of codon use of peptide elongation rate and protein yield is discussed. Finally, the evolutionary aspect of codon selection in mRNA sequences is studied.     

Codon usage and genome composition

Summary The GC levels of codon third positions from 49 genomes coveering a wide phylogenetic range are linearly correlated with the GC levels of the corresponding genomes. Three different relationships have been found: one for prokaryotes and viruses, one for lower eukaryotes, and one for vertebrates. All points not fitting the first relationship can be brought into quasi coincidence with it when plotted against GC levels of coding sequences.     

Codon usage and intragenic position

Data on codon usage bias in E. coli are re-examined with respect to intragenic position. The bias is less extreme near the beginning than in the rest of the gene, particularly in highly expressed genes. This is contrary to the previous finding that there is a linear decline in codon usage bias with position along weakly expressed genes but little or no change in bias along highly expressed genes. The effect is not confined to genes coding for proteins with leader peptides, as suggested earlier (Burns and Beacham, 1985). There is some evidence of a similar but smaller effect in yeast.     

Codon usage and genome evolution

The rates and patterns of evolution at silent sites in codons reveal much about the basic features of molecular evolution. Recent increases in the amount of sequence data available for various species and more precise knowledge of the chromosomal locations of those sequences, coming in particular from genome projects, reveal that some features of molecular evolution vary around the genome.     

Codon usage and gene expression.

The hypothesis that codon usage regulates gene expression at the level of translation is tested. Codon usage of Escherichia coli and phage lambda is compared by correspondence analysis, and the basis of this hypothesis is examined by connecting codon and tRNA distributions to polypeptide elongation kinetics. Both approaches indicate that if codon usage was random tRNA limitation would only affect the rarest tRNA species. General discrimination against their cognate codons indicates that polypeptide elongation rates are maintained constant. Thus, differences in expression of E. coli genes are not a consequence of their variable codon usage. The preference of codons recognized by the most abundant tRNAs in E. coli genes encoding abundant proteins is explained by a constraint on the cost of proof-reading.     

Codon catalog usage and the genome hypothesis.

Frequencies for each of the 61 amino acid codons have been determined in every published mRNA sequence of 50 or more codons. The frequencies are shown for each kind of genome and for each individual gene. A surprising consistency of choices exists among genes of the same or similar genomes. Thus each genome, or kind of genome, appears to possess a "system" for choosing between codons. Frameshift genes, however, have widely different choice strategies from normal genes. Our work indicates that the main factors distinguishing between mRNA sequences relate to choices among degenerate bases. These systematic third base choices can therefore be used to establish a new kind of genetic distance, which reflects differences in coding strategy. The choice patterns we find seem compatible with the idea that the genome and not the individual gene is the unit of selection. Each gene in a genome tends to conform to its species' usage of the codon catalog; this is our genome hypothesis.     

Codon usage and amino acid usage influence genes expression level

Highly expressed genes in any species differ in the usage frequency of synonymous codons. The relative recurrence of an event of the favored codon pair (amino acid pairs) varies between gene and genomes due to varying gene expression and different base composition. Here we propose a new measure for predicting the gene expression level, i.e., codon plus amino bias index (CABI). Our approach is based on the relative bias of the favored codon pair inclination among the genes, illustrated by analyzing the CABI score of the Medicago truncatula genes. CABI showed strong correlation with all other widely used measures (CAI, RCBS, SCUO) for gene expression analysis. Surprisingly, CABI outperforms all other measures by showing better correlation with the wet-lab data. This emphasizes the importance of the neighboring codons of the favored codon in a synonymous group while estimating the expression level of a gene.     

Comparative proteome analysis of Staphylococcus aureus biofilm and planktonic cells and correlation with transcriptome profiling

Pathogenic staphylococci can form biofilms in which they show a higher resistance to antibiotics and the immune defense system than their planktonic counterparts, which suggests that the cells in a biofilm have an altered metabolic activity. Here, 2-D PAGE was used to identify secreted, cell wall-associated and cytoplasmic proteins expressed in Staphylococcus aureus after 8 and 48 h of growth. The proteins were separated at pH ranges of 4-7 or 6-11. The protein patterns revealed significant differences in 427 protein spots; from these, 258 non-redundant proteins were identified using ESI-MS/MS. Biofilm cells expressed higher levels of proteins associated with cell attachment and peptidoglycan synthesis, and in particular fibrinogen-binding proteins. Enzymes involved in pyruvate and formate metabolism were upregulated. Furthermore, biofilm cells expressed more staphylococcal accessory regulator A protein (SarA), which corroborates the positive effect of SarA on the expression of the intercellular adhesion operon ica and biofilm growth. In contrast, proteins, such as proteases and particularly immunodominant antigen A (IsaA) and staphylococcal secretory antigen (SsaA), were found in lower amounts. The RNA expression profiling largely supports the proteomic data. The results were mapped onto KEGG pathways.     

Codon usage in Tetrahymena and other ciliates

Codon usage in ciliates was examined by analyzing the coding regions of 22 ciliate genes corresponding to a total of 26,142 nucleotides (8,714 codons). It was found that Tetrahymena, Paramecium and the hypotrichs (Oxytricha and Stylonychia) differed in which synonymous codons were used most frequently by their genes. In fact, the codon choices in highly expressed Tetrahymena genes were more similar to those of yeast genes than those of Paramecium genes. The ciliates do not appear to have unusually strong biases in codon usage frequency when compared to other protists such as yeast. The analysis of the Tetrahymena genes indicated that genes which are highly expressed during normal cell growth have a stronger bias towards using the "preferred" codons than those expressed at lower levels during growth or for brief periods during processes such as conjugation. This conforms to what is found in other protists.     

Codon usage and secondary structure of mRNA

The specific codon usage pattern of the repetitive unit nucleotide sequence of silk fibroin mRNA suggests that selection has operated on the codon usage to optimize the secondary structure characteristic of the mRNA. The correlation between the stability map of local secondary structure of type I collagen mRNA and the codon usage pattern and the translation rate of the collagen is also implied.     

Codon usage and base composition inRickettsia prowazekii

Codon usage and base composition in sequences from the A + T-rich genome ofRickettsia prowazekii, a member of the alpha Proteobacteria, have been investigated. Synonymous codon usage patterns are roughly similar among genes, even though the data set includes genes expected to be expressed at very different levels, indicating that translational selection has been ineffective in this species. However, multivariate statistical analysis differentiates genes according to their G + C contents at the first two codon positions. To study this variation, we have compared the amino acid composition patterns of 21R. prowazekii proteins with that of a homologous set of proteins fromEscherichia coli. The analysis shows that individual genes have been affected by biased mutation rates to very different extents: genes encoding proteins highly conserved among other species being the least affected. Overall, protein coding and intergenic spacer regions have G + C content values of 32.5% and 21.4%, respectively. Extrapolation from these values suggests thatR. prowazekii has around 800 genes and that 60–70% of the genome may be coding. Correspondence to: S.G.E. Andersson