H-NS Family Proteins Drastically Change Their Targets in Response to the Horizontal Transfer of the Catabolic Plasmid pCAR1

Transcriptional heterogeneity of catabolic genes on the plasmid pCAR1 causes host-specific carbazole degradation

To elucidate why plasmid-borne catabolic ability differs among host bacteria, we assessed the expression dynamics of the Pant promoter on the carbazole-degradative conjugative plasmid pCAR1 in Pseudomonas putida KT2440(pCAR1) (hereafter, KTPC) and Pseudomonas resinovorans CA10. The Pant promoter regulates the transcription of both the car and ant operons, which are responsible for converting carbazole into anthranilate and anthranilate into catechol, respectively. In the presence of anthranilate, transcription of the Pant promoter is induced by the AraC/XylS family regulator AntR, encoded on pCAR1. A reporter cassette containing the Pant promoter followed by gfp was inserted into the chromosomes of KTPC and CA10. After adding anthranilate, GFP expression in the population of CA10 showed an unimodal distribution, whereas a small population with low GFP fluorescence intensity appeared for KTPC. CA10 has a gene, antRCA, that encodes an iso-functional homolog of AntR on its chromosome. When antRCA was disrupted, a small population with low GFP fluorescence intensity appeared. In contrast, overexpression of pCAR1-encoded AntR in KTPC resulted in unimodal expression under the Pant promoter. These results suggest that the expression of pCAR1-encoded AntR is insufficient to ameliorate the stochastic expression of the Pant promoter. Raman spectra of single cells collected using deuterium-labeled carbazole showed that the C-D Raman signal exhibited greater variability for KTPC than CA10. These results indicate that heterogeneity at the transcriptional level of the Pant promoter due to insufficient AntR availability causes fluctuations in the pCAR1-borne carbazole-degrading capacity of host bacterial cells.IMPORTANCEHorizontally acquired genes increase the competitiveness of host bacteria under selective conditions, although unregulated expression of foreign genes may impose fitness costs. The "appropriate" host for a plasmid is empirically known to maximize the expression of plasmid-borne traits. In the case of pCAR1-harboring Pseudomonas strains, P. resinovorans CA10 exhibits strong carbazole-degrading capacity, whereas P. putida KT2440 harboring pCAR1 exhibits low degradation capacity. Our results suggest that a chromosomally encoded transcription factor affects transcriptional and metabolic fluctuations in host cells, resulting in different carbazole-degrading capacities as a population. This study may provide a clue for determining appropriate hosts for a plasmid and for regulating the expression of plasmid-borne traits, such as the degradation of xenobiotics and antibiotic resistance.

Machine-learning from Pseudomonas putida KT2440 transcriptomes reveals its transcriptional regulatory network

Bacterial gene expression is orchestrated by numerous transcription factors (TFs). Elucidating how gene expression is regulated is fundamental to understanding bacterial physiology and engineering it for practical use. In this study, a machine-learning approach was applied to uncover the genome-scale transcriptional regulatory network (TRN) in Pseudomonas putida KT2440, an important organism for bioproduction. We performed independent component analysis of a compendium of 321 high-quality gene expression profiles, which were previously published or newly generated in this study. We identified 84 groups of independently modulated genes (iModulons) that explain 75.7% of the total variance in the compendium. With these iModulons, we (i) expand our understanding of the regulatory functions of 39 iModulon associated TFs (e.g., HexR, Zur) by systematic comparison with 1993 previously reported TF-gene interactions; (ii) outline transcriptional changes after the transition from the exponential growth to stationary phases; (iii) capture group of genes required for utilizing diverse carbon sources and increased stationary response with slower growth rates; (iv) unveil multiple evolutionary strategies of transcriptome reallocation to achieve fast growth rates; and (v) define an osmotic stimulon, which includes the Type VI secretion system, as coordination of multiple iModulon activity changes. Taken together, this study provides the first quantitative genome-scale TRN for P. putida KT2440 and a basis for a comprehensive understanding of its complex transcriptome changes in a variety of physiological states.

Plasmid-mediated catabolism for the removal of xenobiotics from the environment

The large-scale application of xenobiotics adversely affects the environment. The genes that are present in the chromosome of the bacteria are considered nonmobile, whereas the genes present on the plasmids are considered mobile genetic elements. Plasmids are considered indispensable for xenobiotic degradation into the contaminated environment. In the contaminated sites, bacteria with plasmids can transfer the mobile genetic element into another strain. This mechanism helps in spreading the catabolic genes into the bacterial population at the contaminated sites. The indigenous microbial strains with such degradative plasmids are important for the bioremediation of xenobiotics. Environmental factors play a critical role in the conjugation efficiency, which is involved in the bioremediation of the xenobiotics at the contaminated sites. However, there is still a need for more research to fill in the gaps regarding plasmids and their impact on bioremediation. This review explores the role of bacterial plasmids in the bioremediation of xenobiotics from contaminated environments.