Seawater-Regulated Genes for Two-Component Systems and Outer Membrane Proteins in Myxococcus

Abstract

When salt-tolerant Myxococcus cells are moved to a seawater environment, they change their growth, morphology, and developmental behavior. Outer membrane proteins and signal transduction pathways may play important roles in this shift. Chip hybridization targeting the genes predicted to encode 226 twocomponent signal transduction pathways and 74 outer membrane proteins of M. xanthus DK1622 revealed that the expression of 55 corresponding genes in the salt-tolerant strain M. fulvus HW-1 was significantly modified (most were downregulated) by the presence of seawater. Sequencing revealed that these seawater-regulated genes are highly homologous in both strains, suggesting that they have similar roles in the lifestyle of Myxococcus. Seven of the genes that had been reported in M. xanthus DK1622 are involved in different cellular processes, such as fruiting body development, sporulation, or motility. The outer membrane (Om) gene Om031 had the most significant change in expression (downregulated) in response to seawater, while the twocomponent system (Tc) gene Tc105 had the greatest increase in expression. Their homologues MXAN3106 and MXAN4042 were knocked out in DK1622 to analyze their functions in response to changes in salinity. In addition to having increased salt tolerance, sporulation of the MXAN3106 mutant was enhanced compared to that of DK1622, whereas mutating gene MXAN4042 produced contrary results. The results indicated that the genes that are involved in the cellular processes that are significantly changed in response to salinity may also be involved the salt tolerance of Myxococcus cells. Regulating the expression levels of these multifunctional genes may allow cells to quickly and efficiently respond to changing conditions in coastal environments.Isolation techniques based on the formation of fruiting body structures reveal that myxobacteria primarily inhabit various terrestrial environments (5, 20). Salt-tolerant fruiting myxobacterial strains from coastal environments can also be found if the enriching medium is prepared with diluted seawater (16). The salt-tolerant myxobacterial strains exhibit significant shifts in growth, development, and cellular behavior in response to the presence or absence of seawater. For example, when grown in medium with yeast cells as the only nutrient source, the salt-tolerant strain Myxococcus fulvus HW-1 requires a high cell concentration to initiate growth if the cells were collected from seawater-free cultures; the threshold number of cells required for growth decreases if the cells were from seawatercontaining cultures (31). On nutrient-deficient plates, salt-tolerant Myxococcus strains are able to develop typical fruiting bodies only when the seawater concentration is low or absent (the optimal seawater concentration for the development of fruiting bodies varies from 0% to 60% for different salt-tolerant Myxococcus strains) (27). Although the development process is inhibited by high concentrations of seawater, the myxospores can be developed directly from vegetat...

Hdsp, a horizontally transferred gene required for social behavior and halotolerance in salt-tolerant Myxococcus fulvus HW-1

“…Salt-tolerant Myxococcus fulvus strain HW-1 exhibits remarkable changes in the social behavior when cultured in media of different saltwater concentrations. Our previous studies have indicated that maintenance and reinforcement of social behavior are important for HW-1 to adapt to oceanic conditions Zhang et al, 2007;Pan et al, 2009). In this study, the phenotype of the mutant strain YLH0401 indicated that the insertional inactivation of a single horizontally transferred gene led to dramatic phenotypic changes, including dispersed growth, defective fruiting body formation and sporulation, decreased ECM production and S-motility, and seawater intolerance.…”

“…For example, mutation of the S-motility-related gene locus mts (homolog of which in the model strain M. xanthus DK1622 is located at MXAN_1333-1337) led to a simultaneous decrease in salt tolerance . Two seawaterregulated genes-the two component system gene Tc105 (MXAN_4042 in DK1622) and outermembrane protein gene Om031 (MXAN_3016 in DK1622)-are both involved in social behavior and salt tolerance (Pan et al, 2009). In this study, we report an association between the salt-tolerance adaptation and social behavior in a new mutant of M. fulvus HW-1.…”

“…Total RNA was isolated using the Promega SV Total RNA Purification Kit as described by Pan et al (2009). The isolated RNA was treated with the DNA Free Kit (Ambion, Austin, TX, USA) to remove residual genomic DNA.…”

“…It is therefore possible that the putative TPR motifs in Hdsp also have a role in mediating interactions between Hdsp and other proteins. In addition to up-or downregulating the expression of multifunctional genes at transcriptional level to quickly and efficiently respond and acclimate to environmental changes (Pan et al, 2009), the marine salt-tolerant Myxococcus strains probably also horizontally exchange genes to maintain and reinforce social behavior to adapt to oceanic conditions.…”

Two-Component Signal Transduction Systems That Regulate the Temporal and Spatial Expression of Myxococcus xanthus Sporulation Genes

“…On the basis of recent studies, it has been suggested that the Nla6S/Nla6 TCS may be a general regulator of stress-associated genes (59,66). Presumably, the confirmed developmental targets of the Nla6S/Nla6 TCS fall into this category, since they show Nla6-dependent activation 1 h after M. xanthus cells encounter starvation-induced stress (59).…”