Ribosome-binding sites

Very recently, a third hypothesis has been pubHshed. Morita and co-workers [47] have suggested that the rpoH mRNA secondary structure itself acts as a thermosensor. In the absence of heat stress, the rpoH mRNA is folded into a secondary structure that occludes the ribosome binding site and the initiation codon. Upon heat shock, this structure is unfolded allowing ribosome binding and enhanced synthesis. 

In contrast to most mRNAs, which become untranslatable after a temperature downshock, cold shock mRNAs possess a mechanism to form the translation initiation complex at low temperature without cold shock ribosomes. A close inspection of the mRNAs of class I cold shock proteins reveal that they are equipped with an extra ribosome-binding site called the downstream box located within the coding region of their transcript [130]. It would be interesting to know whether introduction of this downstream box into a cellular mRNA would convert it into a transcript which can be transcribed immediately after a cold shock. In the case of the cspA mRNA it has been shown that in the absence of the downstream box the initiation complex cannot be formed at low temperature during the accHmation phase [131]. 

Fig. 24.6 The use of a vector carrying a promoter and adjacent ribosome binding site (RBS) and initiation codon to obtain synthesis of proinsulin from a synthetic gene. The arrow indicates the direction of transcription.

Subcloning of the gene cluster into E. coli [43,136], Pseudomonas [146] and other organisms has been reported. The genes were subcloned independently and expressed in E. coli MZ1 under control of the inducible lambda pL promoter with a lambda ell ribosomal binding site. As a result, the dszC gene has been linked to conversion of DBT to DBT-sulfone [145], dszA to conversion of DBT sulfone to hydroxyphenyl benzene... 

A 7-methylguanosine cap is added to the 5 end while the RNA molecule is still being synthesized. The cap structure serves as a ribosome-binding site and also helps to protect the mRNA chain from degradation. 

The lactose (toe) operon (Fig. 28-7a) includes the genes for jS-galactosidase (Z), galactoside permease (F), and thiogalactoside transacetylase (A). The last of these enzymes appears to modify toxic galactosides to facilitate their removal from the cell. Each of the three genes is preceded by a ribosome binding site (not shown in Fig. 28-7) that independently directs the translation... 

Puromycin. An antibiotic that inhibits polypeptide synthesis by competing with aminoacyl-tRNA for the ribosomal binding site A. 

For the expression of Fabs in E. coli, two polypeptide chains have to be made. To achieve this, there are two strategies. In the monocistronic systems, for example, pComb3, the antibody genes are under control of two promoters and each has its own leader peptide (11). In plasmids like pCESl with a bicistronic Fab operon, both chains are under control of a single promoter, leading to mRNA with two ribosomal binding sites (73). 

Figure 3 The nucleotide sequence and the deduced amino acid sequence of the S. aver-mitilis OMT gene. A potential ribosome binding site is in bold type and double-underlined. The stop codon is indicated by an asterisk. Conserved domains found in methyltransferases are underlined.

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