The origin and evolution of multidomain proteins are driven by diverse processes including fusion/fission, domain shuffling, and alternative splicing. The 20 aminoacyl-tRNA synthetases (AARS) constitute an ancient conserved family of multidomain proteins. The glutamyl-prolyl tRNA synthetase (EPRS) of bilaterian animals is unique among AARSs, containing two functional enzymes catalyzing ligation of glutamate and proline to their cognate transfer RNAs (tRNAs). The ERS and PRS catalytic domains in multiple bilaterian taxa are linked by variable number of helix-turn-helix domains referred to as WHEP-TRS domains. In addition to its canonical aminoacylation activities, human EPRS exhibits a noncanonical function as an inflammation-responsive regulator of translation. Recently, we have shown that the WHEP domains direct this auxiliary function of human EPRS by interacting with an mRNA stem-loop element (interferon-gamma-activated inhibitor of translation [GAIT] element). Here, we show that EPRS is present in the cnidarian Nematostella vectensis, which pushes the origin of the fused protein back to the cnidarian-bilaterian ancestor, 50-75 My before the origin of the Bilateria. Remarkably, the Nematostella EPRS mRNA is alternatively spliced to yield three isoforms with variable number and sequence of WHEP domains and with distinct RNA-binding activities. Whereas one isoform containing a single WHEP domain binds tRNA, a second binds both tRNA and GAIT element RNA. However, the third isoform contains two WHEP domains and like the human ortholog binds specifically to GAIT element RNA. These results suggest that alternative splicing of WHEP domains in the EPRS gene of the cnidarian-bilaterian ancestor gave rise to a novel molecular function of EPRS conserved during metazoan evolution.

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Two fresh-water sponge species, Ephydatiafluviatilis and Spongilla alba, were grown from gemmules in the laboratory. A system incorporating a continuous flow of filtered habitat water and live bacteria from a chemostat culture as a food source were used. Experiments with this system demonstrated a relationship between the concentration of bacteria and sponge growth rate. Because the continuous flow of water eliminates the effects of substances released by sponges and growth rate can be predicted for a given bacterial concentration, this system permits experimental studies which were not feasible in the past.

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The freshwater sponge, Ephydatiafluviatilis (Porifera: Spongillidae), was maintained in a continuous-Pow laboratory culture system under several conditions of calcium ion (Ca++) concentration and salinity. Experimental results suggest that sponge growth rate increases with increasing Ca+ + concentration, that sponge growth rate decreases with increasing salinity, and that the negative effect of higher salinity can be overcome by increasing Ca + + concentration. The experimental results correlate well with field observations on the effects of salinity and Ca + + on the distribution of E. fluviatilis.