Cloning and Functional Expression of a Soluble Form of Kynurenine/α -Aminoadipate Aminotransferase from Rat Kidney

5-Noncoding sequences have been tabulated for 211 messenger RNAs from higher eukaryotic cells. The 5'-proximal AUG triplet serves as the initiator codon in 95% of the mRNAs examined. The most conspicuous conserved feature is the presence of a purine (most often A) three nucleotides upstream from the AUG initiator codon; only 6 of the mRNAs in the survey have a pyrimidine in that position. There is a predominance of C in positions -1, -2, -4 and -5, just upstream from the initiator codon. The sequence CCAGCCAUG (G) thus emerges as a consensus sequence for eukaryotic initiation sites. The extent to which the ribosome binding site in a given mRNA matches the -1 to -5 consensus sequence varies: more than half of the mRNAs in the tabulation have 3 or 4 nucleotides in common with the CCACC consensus, but only ten mRNAs conform perfectly.

Representative samples of mitochondrial and chloroplast targeting peptides have been analyzed in terms of amino acid composition, positional amino acid preferences and amphiphilic character. No highly conserved 'homology blocks' are found in either class of topogenic sequence. Mitochondrial-matrix-targeting peptides are composed of two domains with different amphiphilic properties. Arginine is frequently found either at position -10 or -2 relative to the cleavage site, suggesting that some targeting peptides may be cleaved twice in succession by two different matrix proteases. In stroma-targeting chloroplast transit peptides three distinct regions are evident: an uncharged amino-terminal domain, a central domain lacking acidic residues and a carboxy-terminal domain with the potential to form an amphiphilic beta-strand. Targeting peptides that route proteins to the mitochondrial intermembrane space or the lumen of chloroplast thylakoids have a mosaic design with an amino-terminal matrix- or stroma-targeting part attached to a carboxy-terminal extension that shares many characteristics with secretory signal peptides.