LRP protein competes with the Dam methylase for access to two 5′‐GATC‐3′ sites in the pap and pef fimbrial operons of uropathogenic E. coli and Salmonella, respectively. This outcome of competition decides if the Dam sites will be methylated or not and this, in turn, determines if the fimbrial structural genes will be transcribed or not. The result is a stochastic switch that is reset by the synthesis of hemimethylated DNA during chromosome (pap) or virulence plasmid (pef) replication (Hernday et al. 2002; Nicholson and Low 2000). In contrast, LRP acts as a directionality determinant at the invertible fimS genetic switch that governs the phase‐variable expression of type 1 fimbriae in E. coli, an inversion event that is catalysed by two tyrosine integrases, FimB and FimE (Corcoran and Dorman 2009; Kelly et al. 2006). In the case of pap/pef, LRP is acting as a DNA‐binding protein in competition with a DNA modifying enzyme; in the case of fimS, LRP's ability to shape DNA is likely to be influencing the directionality of the On/Off invertible genetic switch. It also plays a role in virulence in M. tuberculosis by modulating the innate immune response of macrophage (Liu and Cai 2018). These examples illustrate the versatility of the LRP protein and its ability to influence diverse systems by different molecular mechanisms.
1.54 Small, Acid‐soluble Spore Proteins, SASPs
Aerobic and anaerobic spore‐forming bacteria rely on small, acid‐soluble spore proteins (SASPs) to protect their DNA from damage during the long (or very long) periods that may elapse between sporulation and spore germination. Most research on SASPs has been concerned with those produced by Bacillus spp. and Clostridium spp., aerobic and anaerobic organisms, respectively (Setkow 2007). The SASPs fall into two broad groups, the α/β type and the γ type. Their genes are transcribed with the G sigma factor (Nicholson et al. 1989), protect the genomic DNA in the spore, and after germination they are cannibalised as a source of amino acids by the emerging bacterial cell (Hackett and Setlow 1988; Setlow 1988). SASPs are specialists in that they are expressed specifically to accompany the genome during its period of storage in the spore and are degraded during germination; they do not have physiological roles in vegetative cells, yet they exhibit properties that are shared with NAPs. For example, they stiffen DNA, and eliminate DNA bends, they increase the persistence length of DNA and introduce supertwists into relaxed or nicked circular DNA (Griffith et al. 1994; Nicholson et al. 1990), including plasmids from spores (Nicholson and Setlow 1990). When cloned ssp genes expressing SASPs are introduced to E. coli, they induce nucleoid condensation; however, they also induce mutagenesis and cell killing (Setlow et al. 1991, 1992). The mutations accompanied expression of the B. subtilis SspC SASP and required RecA and Pol V, suggesting that the effects followed the arrest of replication forks in growing E. coli; a derivative of SspC that was deficient in DNA binding failed to elicit these deleterious effects when expressed in E. coli (Setlow et al. 1992). Overall, the effect of SASP expression in E. coli was to cause the Gram‐negative bacterium to assume some of the characteristics of a sporulating organism (Setlow et al. 1991).
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