Clostridia belong to those bacteria which are considered as obligate anaerobe, 
e.g. oxygen is harmful or lethal to these bacteria. Nevertheless, it is known 
that they can survive limited exposure to air, and often eliminate oxygen or 
reactive derivatives via NAD(P)H-dependent reduction. This system does 
apparently contribute to survival after oxidative stress, but is insufficient to 
establish long-term tolerance of aerobic conditions. Here we show that 
manipulation of the regulatory mechanism of this defence mechanism can trigger 
aerotolerance in the obligate anaerobe Clostridium acetobutylicum. Deletion of a 
peroxide repressor (PerR)-homologous protein resulted in prolonged 
aerotolerance, limited growth under aerobic conditions and rapid consumption of 
oxygen from an aerobic environment. The mutant strain also revealed higher 
resistance to H2O2 and activities of NADH-dependent scavenging of H2O2 and 
organic peroxides in cell-free extracts increased by at least one order of 
magnitude. Several genes encoding the putative enzymes were upregulated and 
identified as members of the clostridial PerR regulon, including the heat shock 
protein Hsp21, a reverse rubrerythrin which was massively produced and became 
the most abundant protein in the absence of PerR. This multifunctional protein 
is proposed to play the crucial role in the oxidative stress defence.