The PrfA thermoregulator UTR is an RNA thermometer

RNA thermometer

An RNA thermometer is a temperature-sensitive non-coding RNA molecule which regulates gene expression. RNA thermometers often regulate genes required during either a heat shock or cold shock response, but have been implicated in other regulatory roles such as in pathogenicity and starvation.In...

 found in the 5' UTR of the prfA gene. In Listeria monocytogenes, virulence genes are maximally expressed at 37°C (human body temperature

Normal human body temperature

Normal human body temperature, also known as normothermia or euthermia, is a concept that depends upon the place in the body at which the measurement is made, and the time of day and level of activity of the person...

) but are almost silent at 30°C. The genes are controlled by PrfA, a transcriptional activator whose expression is thermoregulated. It has been shown that the untranslated mRNA (UTR) preceding prfA, forms a secondary structure

Secondary structure

In biochemistry and structural biology, secondary structure is the general three-dimensional form of local segments of biopolymers such as proteins and nucleic acids...

, which masks the ribosome binding region. It is thought that at 37°C, the hairpin

Stem-loop

Stem-loop intramolecular base pairing is a pattern that can occur in single-stranded DNA or, more commonly, in RNA. The structure is also known as a hairpin or hairpin loop. It occurs when two regions of the same strand, usually complementary in nucleotide sequence when read in opposite directions,...

structure 'melts' and the SD sequence is unmasked.