Differential codon usage: a safeguard against inappropriate expression of specialized genes?

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.