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Translation rate modification by preferential codon usage: intragenic position effects.

We present a model for calculating the protein production rate as a function of the translation rate. The model takes into account that the elongation rate along an mRNA molecule is non-uniform as a result of different tRNA availabilities for different codons. Initiation of ribosomes on an mRNA is normally the rate-limiting step in the translation process, and blocking of the initiation site can be avoided if the codons closest to this site allow fast translation by the ribosome. Hence, different selective forces may act on the choice of synonymous codons in the initiation region than elsewhere on a given mRNA. We show that the elongation rate along the whole mRNA influences the production rate of abundant proteins, whereas only the elongation rate in the initiation region is of importance for the production rate of rare proteins. We also present an analysis of the codon distribution along known mRNAs coding for abundant and rare proteins.

Bacterial Proteins

Preliminary indication of unusual codon usage in the DNA coding sequence of the attachment protein of Mycoplasma pneumoniae.

From a Mycoplasma pneumoniae genomic library, three recombinant clones encoding approximately one-third of the attachment (P1) gene were identified. P1 fusion proteins expressed by these clones in Escherichia coli were found to be much smaller than expected from the sizes of the cloned DNA fragments. Nucleotide sequence analysis revealed the presence of UGA codons in the open reading frames of two of the clones, explaining the incomplete translation of the inserts. Sequencing data further revealed that two of the recombinant clones did have similar but not identical carboxyl-end sequences. This finding suggests the existence of more than one genomic DNA sequence coding for the 3'-end of the P1 gene. Potential transcriptional regulatory sequences, a possible termination signal at the 3'-end of the P1 gene and possible promoter-like structures, have been recognized.

Amino Acid Sequence

Strong homology between the small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase of two species of Acetabularia and the occurrence of unusual codon usage.

Amino acid sequences of the small subunit of ribulose-1,5-bisphosphate carboxylase (SSU) of Acetabularia cliftonii and A. mediterranea were derived from five cDNA sequences of each of the two species of algae and by direct amino acid sequence determination of the isolated protein. An homology of more than 96% between the proteins indicates the close relationship between the two algae. All ten cDNAs in the reading frame display the termination codons TAA and/or TAG at various positions, which seem to code for the amino acid glutamine when compared with the amino acid sequence from the mature protein. This is reminiscent of proteins from ciliates where TAA and TAG also code for glutamine.

Acetabularia