The first directs expression of the immediate upstream gene rpsO, and the second is positioned in the rpsO-pnp intergenic region (Portiers & Reginer, 1984). Irrespective of the transcriptional start site, the pnp mRNA is vulnerable to cleavage by endoribonuclease RNase III at positions

within 75 nucleotides upstream the pnp ORF, which in turn initiates degradation of the pnp mRNA by PNPase itself (Portier et al., 1987). Upon a cold shock, the pnp mRNA becomes stabilized allowing enhanced expression of PNPase (Beran & Simons, 2001). In enterobacteria, pnp is followed by nlpI (Blattner et al., 1997; McClelland et al., 2001; Nie et al., 2006). For E. coli, NlpI has been shown to be a lipoprotein (Ohara et al., 1999). We recently demonstrated that PNPase and NlpI posed opposing effect on biofilm formation in S. Typhimurium Ku 0059436 at decreased growth temperature (Rouf et al., 2011). Experiments that followed here demonstrate that mutational inactivation of pnp in S. Typhimurium results in an expected restricted growth at 15 °C. In addition, the experiments showed that pnp transcripts continued into nlpI and that nonpolar pnp mutations increased nlpI expression. Although S. Typhimurium pnp and nlpI are separated

Copanlisib purchase by 109 base pairs, the promoter prediction software bprom (www.Softberry.com) failed to define any tentative nlpI promoter within this intergenic region (data not shown). Combined with the gene expression analysis, this strongly suggests that pnp and nlpI form an operon and implies that nlpI is subject to the same post-translational regulation of pnp. However, we cannot formally exclude potential nlpI promoters within pnp. The co-transcription of pnp and nlpI led us to detail whether, and to what extent, NlpI contributed to cold acclimatization. The data presented in this study demonstrate that nlpI does indeed functionally act as a cold shock gene in concert with, but independently of, pnp. Evidence to support includes the observation that two of the the three pnp mutants applied in this study had enhanced expression of nlpI, whilst the third had unaffected nlpI mRNA levels compared

to the wild type, yet all three mutants showed a very similar defect for growth at 15 °C. In addition, a pnp–nlpI double mutant had more restricted growth at 15 °C compared to either single mutant, whilst cloned pnp and nlpI enhanced the replication of all the respective mutants at 15 °C (Figs 4b and 5). The nlpI gene is adjacent to csdA/deaD in the genomes of enterobacteria (Blattner et al., 1997; McClelland et al., 2001; Nie et al., 2006). The csdA gene encodes for an alternative RNA helicase that in E. coli also contributes to cold acclimatization (Turner et al., 2007). In S. Typhimurium, the homologue for csdA is defined as deaD. Deleting deaD in S. Typhimurium resulted in a cold-sensitive growth phenotype. However, we could not trans-complement the cold-restricted growth of the deaD mutant phenotype with either pnp or nlpI.