Maintaining correct cellular function is a fundamental biological process for 
all forms of life. A critical aspect of this process is the maintenance of 
protein homeostasis (proteostasis) in the cell, which is largely performed by a 
group of proteins, referred to as the protein quality control (PQC) network. 
This network of proteins, comprised of chaperones and proteases, is critical for 
maintaining proteostasis not only during favourable growth conditions, but also 
in response to stress. Indeed proteases play a crucial role in the clearance of 
unwanted proteins that accumulate during stress, but more importantly, in the 
activation of various different stress response pathways. In bacteria, the cells 
response to stress is usually orchestrated by a specific transcription factor 
(sigma factor). In Escherichia coli there are seven different sigma factors, 
each of which responds to a particular stress, resulting in the rapid expression 
of a specific set of genes. The cellular concentration of each transcription 
factor is tightly controlled, at the level of transcription, translation and 
protein stability. Here we will focus on the proteolytic regulation of two sigma 
factors (σ(32) and σ(S)), which control the heat and general stress response 
pathways, respectively. This review will also briefly discuss the role 
proteolytic systems play in the clearance of unwanted proteins that accumulate 
during stress.