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...be more stable during stationary phase. This was found to aid the survival of bacteria in this phase. Such proteins include ribosome modulation factor (RMF), YfiA and YhbH. Examining the influence of RMF on the survival of E. coli under heat, acid and osmotic stress showed that it was important for bacterial viability under these environmental pressures. However, the mechanism by which this protein exerts its effect has not been fully elucidated. The present work reviews the involvement of ribosomes in determining cell behaviour during stress. It focuses on the action of the ribosome-associated proteins and their role in inactivating ribosomes for preserving their integrity and aiding cell survival under stress.
Keywords: bacterial ribosome, cellular response to environmental stress, RMF, YfiA, YhbH
Ribosomes and stress
The ability of bacteria to express adaptive mechanisms to cope with environmental stress has received a lot of research and speculation over last few decades. These mechanisms involve various cell structures and networks of orchestrated behaviour. The ribosome is one cellular component whose reaction to stress and whose effect on cell survival have been extensively studied. An important reason for this interest is the involvement of ribosomes in protein synthesis, being the stage at which genetic codons are translated into proteins. The importance of ribosomes also arises from their representing approximately 40% of the mass of rapidly-growing cells and from that most of cellular energy is devoted for their synthesis and assembly. (1) The functional ribosomal unit in bacteria is designated 70S (S refers to Svedberg units for sedimentation rate) which consists of two subunits: 30S and 50S (Figure 1). Proteins and RNA constitute the major components of ribosomes; there are more than 50 proteins and three RNA species (23S, 16S and 5S) in E. coli ribosomes (2).
[FIGURE 1 OMITTED]
Early studies of the involvement of ribosomes in environmental stress showed that the degradation of ribosomal components, particularly RNA, correlated well with cell death during heat, cold and starvation (3-5). This suggested an involvement of ribosome damage in cell death. This concept was supported by observations that the presence of factors stabilising ribosomes, such as [Mg.sup.2+], aided bacterial survival. Conversely, ribosome-destabilising agents, such as EDTA, increased cell death under stress (3-4).
With advances in molecular cell analysis, it was possible to provide closer visualisation of cell damage and to observe that rRNA species and ribosomal subunits are not equally vulnerable to stress. It was shown that 16S rRNA and 30S subunits are more sensitive to heat stress than 23S rRNA and 50S subunits, respectively (6). Depriving Salmonella ser. Typhimurium of a metabolisable carbon source and growing Listeria monocytogenes under sub-lethal salt stress were observed to increase their tolerance to heat (7). This phenomenon, termed "cross protection", was found to be associated with increased stability of 16S rRNA and reduced ribosomal damage following exposure to heat. It has also been suggested that ribosomes were involved in the increased cell sensitivity to heat shock after being subjected to abrupt temperature downshift. Under these conditions, cold shock caused a decrease in thermal stability of 50S and 70S units that was proposed to be reflected in cell vulnerability to heat (8).
However, ribosome stability cannot be considered as the sole factor determining bacterial behaviour during stress. For example, although cell death correlated to ribosome damage under heat and cold stress, there was an initial stage during exposure to stress where there was rapid ribosome disintegration associated with no decline in cell viability (3,4). It was suggested that ribosome damage was not a direct cause of cell death, which might have been the result of a rapid increase in the endogenous pool of components caused by RNA degradation. On the other hand, Niven et al. (9) showed that there was a reduction in ribosomal numbers and a decline in their stability on exposing E. coli to high pressure. While these were improved on eliminating pressure and incubating cultures at optimum growth conditions, bacterial viability continued to be lost.
Following interesting observations by VanBogelen and Neidhardt in 1990 (10), ribosomes were suggested to be sensors to heat and cold shocks in Escherichia coli. It was found that the addition of antibiotics targeting ribosomes induced the expression of heat and cold shock proteins. These proteins are synthesized in bacteria under normal growth conditions, but their expression is induced significantly on expose to rapid temperature shift. They are presumed to aid cell survival by acting as chaperones interacting with other molecules to prevent or overcome temperature-induced damage (11). It was interesting to observe that the patterns of heat or cold shock proteins induced by antibiotics simulated those induced by temperature shifts that the induction extent in the former case depended on drug dose in a similar way to the induction dependency on the severity of temperature shock. Zhang et al. (12) have revisited this phenomenon in Bacillus subtilis. They observed the involvement of the L11 ribo somal protein in the activation of sigma factor B, the general stress sigma factor in B. subtilis, following exposure to stress and concluded that ribosomes could serve as a sensor for most stresses encountered by this organism.
Not only can heat shock proteins aid cell survival by functioning as chaperones, but they were also found to be involved in facilitating ribosomal translation during stress. For example, it was observed...
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