A Cold-Sensitive Listeria monocytogenes Mutant Has a Transposon Insertion in a Gene Encoding a Putative Membrane Protein and Shows Altered (p)ppGpp Levels

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Abstract
A cold-sensitive Listeria monocytogenes mutant designated cld-14 was obtained by transposon Tn 917 mutagenesis. The gene interrupted by Tn 917 in cld-14 was the L. monocytogenes LMOf2365_ 1485 homolog, which exhibits 45.7% homology to the Bacillus subtilis yqfF locus. LMOf2365_ 1485 , here designated pgpH , encodes a putative integral membrane protein with a predicted molecular mass of 81 kDa. PgpH is predicted to contain a conserved N-terminal signal peptide sequence, seven transmembrane helices, and a hydrophilic C terminus, which likely extends into the cytosol. The Tn 917 insertion in pgpH is predicted to result in production of a premature polypeptide truncated at the fifth transmembrane domain. The C terminus of PgpH, which is probably absent in cld-14, contains a highly conserved HD domain that belongs to a metal-dependent phosphohydrolase family. Strain cld-14 accumulated higher levels of (p)ppGpp than the wild type accumulated, indicating that the function of PgpH may be to adjust cellular (p)ppGpp levels during low-temperature growth. The cld-14 pgpH + complemented strain was able to grow at a low temperature, like the parent strain, providing direct evidence that the activity of PgpH is important in low-temperature adaptation. Because of its predicted membrane location, PgpH may play a critical role in sensing the environmental temperature and altering cellular (p)ppGpp levels to allow the organism to adapt to low temperatures.