Mycoplasma Gene Expression in Escherichia coli

The expression of mycoplasma genes in Escherichia coli offers some interesting and unusual challenges compared to the expression of most other bacterial or eukaryotic cloned genes. For instance, the members of the genera Mycoplasma, Ureaplasma , and Spiroplasma all appear to utilize UGA (opal stop) codons as tryptophan coding codons (1 ). In addition, UGA seems to be the most common tryptophan coding codon in these genera as well, increasing the likelihood that many mycoplasma genes contain at least one or more UGA codons. This results in premature truncation of polypeptides during translation in normal E. coli hosts leading directly to the lack of expression of cloned mycoplasma genes, and the failure to identify cloned genes in genomic libraries using monospecific immunoreagents, such as monoclonal antibodies (MAbs). Using hyperimmune antisera, it has been possible to identify some cloned genes because of the probability of having antibodies directed against amino-terminal regions of the truncated polypeptides, and obviously, gene sequences lacking UGA codons are easily identified. This unusual codon usage, however, does prevent a systematic identification and analysis of the genes responsible for the complete antigenic repertoire of the mycoplasma genome.