Temporal and spatial expression of ammonium transporter genes during growth and development of Dictyostelium discoideum

Ammonia is an important signaling molecule involved in the regulation of development in Dictyostelium. During aggregation, ammonia gradients are established, and the ammonia concentration in the immediate environment or within a particular cell throughout development may vary. This is due to the rate of cellular ammonia production, its rate of loss by evaporation to the atmosphere or by diffusion into the substratum, and perhaps to cellular transport by ammonium transporters (AMTs). Recent efforts in genome and cDNA sequencing have identified three ammonium transporters in Dicyostelium. In addition to physically altering the levels of ammonia within cells, AMTs also may play a role in ammonia signaling. As an initial step in identifying such a function, the temporal and spatial expression of the three amt genes is examined. RT-PCR demonstrates that each of the three amt mRNAs is present and relatively constant throughout growth and development. The spatial expression of these three amt genes is examined during multiple stages of Dictyostelium development using in situ hybridization. A distinct and dynamic pattern of expression is seen for the three genes. In general, amtA is expressed heavily in pre-stalk cells in a dynamic way, while amtB and amtC are expressed in pre-spore regions consistently throughout development. AmtC also is expressed in the most anterior tip of fingers and slugs, corresponding to cells that mediate ammonia's effect on the choice between slug migration and culmination. Indeed, amtC null cells have a slugger phenotype, suggesting AmtC functions in the signaling pathway underlying the mechanics of this choice.