نمونه متن انگلیسی مقاله
Recent work has suggested that rare codons are sometimes used for the regulation of specialized gene expression in bacteria. Moreover, the cellular levels of certain tRNAs may fluctuate with growth conditions. Evidence implicating such mechanisms in the control of photosynthesis in Rhodobacter, solventogenesis in Clostridium, sporulation in Streptomyces, and fimbrial phase variation in E. coli is summarized. It is suggested that such mechanisms will prove applicable to the control of numerous additional specialized functions, and that the empirical tools for testing this possibility are currently available.
In 1989, Brinkman et al. noted that eukaryotic proteins such as the human tissue type plasminogen activator, prourokinase, and the gp41 protein of HIV, which have a high content of rare codons in their respective genes, are poorly expressed in E. coli . Moreover, induction of the expression of any one of these heterologous, plasmid-encoded genes was found to inhibit cell division and cause plasmid instability. Most remarkably, when the bacteria were simultaneously provided with a plasmid bearing the dna Ygene, encoding a rare tRNA (tRNA~,~A/AGG), production of the eukaryotic proteins was increased while plasmid stability and cell viability improved . While these observations were of considerable practical significance to the bioengineer, they foreshadowed observations and experiments that would suggest that the use of rare codons for specialized or differentiation-specific functions in bacteria might provide a general mechanism to ensure proper temporal and spatial expression of the encoding genes. Although this hypothesis is still far from established, work in several laboratories has provided indirect evidence suggesting that rare codon usage is of functional significance in restricting or specifying appropriate gene expression. In this minireview I summarize the evidence concerned with this issue and reiterate the suggestion that the complement of tRNAs found in a particular bacterium under one set of growth conditions may differ from that found under another set of growth conditions.