In usual, the selection of the genes of interest for engineering the Escherichia coli (E. coli) bacterium is determined by the following parts. These parts are with interesting properties that would allow cells to perform naturally unattainable functions -

The nucleotide sequence of DNA in E. coli contains a number of units of transcription, known as the operon, which includes a promoter, an operator and one or more structural genes. (Fig. 2) A promoter is a DNA sequence to which the enzyme responsible for transcribing DNA into RNA, or the RNA polymerase binds. An operator is normally a segment between the promoter and the structural genes,4 to which a specific kind of protein such as a repressor or activator may bind as a means to prevent or stimulate transcription respectively. Note that the inducer molecules may bind to repressor to stop the repressor from preventing transcription. Finally, structural genes are the sequences to be transcribed into mRNA molecules. If we visualise the operon as the track of a racing competition, then we may think RNA polymerase as the racer, and repressor as the obstacles, preventing the racer to go through the rest of the track which is the structural genes.

From here, the protein present in cells responsible for translating mRNA into proteins, or the ribosome, binds to the ribosome binding site (RBS) of the mRNA, goes along the mRNA strand, and finally translates it into a polypeptide chain which folds itself into a protein.

References

1 Large-volume transformation with high-throughput efficiency chemically competent cells. Focus 20:2 (1998)