Bacteria secrete siderophores and then the iron-siderophore complex is taken up by bacterial receptors. Whether siderophores can be considered virulence factors is debatable, as non-pathogens also use this mechanism to acquire iron. Indeed, some pathogens can grow using transferrin as a source of iron. Examples in E. coli include sigma 32, for transcription of heat shock genes, and sigma 54 for transcription of genes regulated under nitrogen limitation. Specific recognition of the ribosome binding sites at the 5' end of the mRNA with the 16S RNA of the ribosome.
Translation ends at a stop codon, or non-sense codon.
The genes are thus coordinately expressed, but each contains its own ribosome binding site. Grouping of several operons under the control of a common regulatory protein is a regulon. Polarity occurs when a mutation in an operon effects genes downstream in the operon. For example, a transposon insertion in an operon generally prevents expression of downstream genes. Nonsense codons can be polar because they block translational coupling or promoter mRNA degradation.