Transcriptome profiling reveals the importance of plasmid pSymB for osmoadaptation of Sinorhizobium meliloti

Abstract

In this work, DNA microarrays were used to investigate genome-wide transcriptional responses of Sinorhizobium meliloti to a sudden increase in external osmolarity elicited by addition of either NaCl or sucrose to exponentially growing cultures. A time course of the response within the first 4 h after the osmotic shock was established. We found that there was a general redundancy in the differentially expressed genes after NaCl or sucrose addition. Both kinds of stress resulted in induction of a large number of genes having unknown functions and in repression of many genes coding for proteins with known functions. There was a strong replicon bias in the pattern of the osmotic stress response; whereas 64% of the upregulated genes had a plasmid localization, 85% of the downregulated genes were chromosomal. Among the pSymB osmoresponsive genes, 83% were upregulated, suggesting the importance of this plasmid for S. meliloti osmoadaptation. Indeed, we identified a 200-kb region in pSymB needed for adaptation to saline shock which has a high density of osmoregulated genes.

FIG. 1.

Numbers of differentially expressed genes at different times after addition of 0.3 M NaCl (▪), 0.4 M NaCl (▴), 0.5 M sucrose (□), and 0.7 M sucrose (▵).

FIG. 2.

Distribution of genes upregulated (left diagram) or downregulated (right diagram) after an osmotic upshift for the three S. meliloti 1021 replicons (chromosome, pSymA, and pSymB).

FIG. 3.

Distribution of upregulated (solid bars) and downregulated (open bars) genes in functional categories.

FIG. 4.

Growth curves for Rm1021 (squares), RmF514 (circles), and RmF909 (triangles) in TY (open symbols) or TY containing 0.4 M NaCl (solid symbols). The data are from at least two replicate experiments. A600, optical density at 600 nm.

FIG. 5.

Growth curves for Rm1021 (squares), RmF514 (circles), and RmF909 (triangles) in TY (open symbols) or TY containing 0.5 M NaCl (solid symbols). The arrow indicates the time of application of the osmotic upshift. The data are from at least two replicate experiments. A600, optical density at 600 nm.