# Adaptation of the Marine Bacterium _Shewanella baltica_ to Low Temperature Stress
## 2 Results and Discussion
### Downregulation of a Type II Secretion System in Response to Cold Stress
Scheme of type II secretion system with twin-arginine targeting (Tat) and SecA and signal recognition particle (Sec-SRP) translocons (source: KEGG database). Tripartite heatmap boxes show row Z-score scaled normalized expression values for corresponding genes upon cold stress at 0, 90 and 180 min time points, respectively. Question mark denotes that the gene is not annotated in _S. baltica_ OS185. of the Ffh protein and 4.5S RNA, which mediates co-translational targeting of preproteins [70]. Fully folded proteins in the cytoplasm are transported across the plasma membrane to the periplasm by the Tat system [69] and T2 SS is the only known secretion system for the Tat-delivered exoproteins [65]. Upon cold stress exposure, expression of genes encoding components of the general secretory pathway (Sec) and HlyD family secretion proteins were reported to be either repressed or unaffected in _S. oneidensis_[23], but expression of _tatA_, _tatB_ and _tatC_ genes encoding components of the Tat-dependent protein translocon were shown to be induced by low temperature [23]. In our study, we observed a significant downregulation of expression of the SecB-encoding gene (Shew185_0045) upon cold stress (Figure 4), suggesting that targeting of exoproteins to the Sec translocon for co-translational transport across the inner membrane may be highly temperature-dependent in _S. baltica_. However, we observed a transcriptional downregulation in cold exposed _S. baltica_ only for one component of the Sec-SRP translocon itself, the SecD/F (Shew185_2804), while several other of its components were significantly upregulated. We also found upregulation of the _ffh_ gene expression (Shew185_1253), i.e., of the gene coding for the signal recognition particle protein. Expression of the Tat system components was also slightly affected by low temperature--the TatC (Shew185_0414) encoding gene was upregulated after 90 and 180 min of cold stress, while expression of TatA (Shew185_0416) and TatB (Shew185_0415) encoding genes was downregulated (Figure 4). In our study, expression of the Tat translocon-encoding genes was only slightly regulated upon cold stress at the transcriptional level, but as the proteins delivered to the periplasm via this route are secreted by the T2S system, secretion of these exoproteins may also be affected by the low temperature stress. In summary, our study confirms that cold stress conditions affect protein transport across membranes in _S. baltica_. Transcriptional data suggest that transport of the Sec-SRP-dependent proteins may be limited at least to some extent due to downregulation of the SecB chaperone gene expression, required for appropriate targeting of preproteins to the Sec translocon; downregulation of the entire T2S system expression may result in substantial restriction of the exoprotein secretion. As the T2 SS transport is reduced at the transcriptional level throughout the entire period of our experiment, such downregulation may be considered as both, an early response and an adaptation of _S. baltica_ to low temperature conditions.