Expression and physiological role of three Myxococcus xanthus copper-dependent P1B-type ATPases during bacterial growth and development.

Myxococcus xanthus is a soil-dwelling bacterium that exhibits a complex life cycle comprising social behavior, morphogenesis, and differentiation. In order to successfully complete this life cycle, cells have to cope with changes in their environment, among which the presence of copper is remarkable. Copper is an essential transition metal for life, but an excess of copper provokes cellular damage by oxidative stress. This dual effect forces the cells to maintain a tight homeostasis. M. xanthus encodes a large number of genes with similarities to others reported previously to be involved in copper homeostasis, most of which are redundant. We have identified three genes that encode copper-translocating P(1B)-ATPases (designated copA, copB, and copC) that exhibit the sequence motifs and modular organizations of those that extrude Cu(+). The expression of the ATPase copC has not been detected, but copA and copB are differentially regulated by the addition of external copper. However, while copB expression peaks at 2 h, copA is expressed at higher levels, and the maximum is reached much later. The fact that these expression profiles are nearly identical to those exhibited by the multicopper oxidases cuoA and cuoB suggests that the pairs CuoB-CopB and CuoA-CopA sequentially function to detoxify the cell. The deletion of any ATPase alters the expression profiles of other genes involved in copper homeostasis, such as the remaining ATPases or the Cus systems, yielding cells that are more resistant to the metal.

Differential regulation of six heavy metal efflux systems in the response of Myxococcus xanthus to copper.

Myxococcus xanthus has to cope with changes in its environment during growth and development. Among these factors, the concentration of copper is crucial due to the essential toxic effect of this metal, which forces the cells to maintain a tight homeostasis. The M. xanthus copper response is more complex than that in other bacteria, which is reflected by the different copper sensitivities of growing and developing cells. In the present study, the participation in copper homeostasis of six heavy metal efflux systems encoded in the M. xanthus genome has been examined. Three of these pumps exhibit the signature sequences in transmembrane domain 4 of the Cus systems (Cus1, Cus2, and Cus3), while the other three exhibit the motifs of the Czc systems (Czc1, Czc2, and Czc3). The Cus2 and Cus3 systems are inducible by copper and monovalent metals, functioning as the main copper efflux pumps, while the Cus1 system is implicated in Zn(2+) homeostasis. The Czc systems are also differentially regulated either by divalent metals but not by copper (Czc1), by copper and divalent metals (Czc2), or by starvation (Czc3). The differential regulation of these six efflux systems ensures the proper completion of the M. xanthus life cycle in an environment with fluctuating concentrations of copper and other metals.