The interaction of the chloroplast elongation factor Tu (EF-Tuchl) from Euglena gracilis with guanine nucleotides and aminoacyl-tRNA has been investigated. The apparent dissociation constant at 37 degrees C for the EF-Tuchl X GDP complex is about 3 X 10(-7) M and for the EF-Tuchl X GTP complex, it is about 1 order of magnitude higher. The sulfhydryl modifying reagent N-ethylmaleimide severely inhibits the polymerization activity of Euglena EF-Tuchl. In the presence of N-ethylmaleimide, the dissociation constant for the modified EF-Tuchl X GDP complex is increased by an order of magnitude. Conversely, both GDP and GTP protect EF-Tuchl from the modification. The polymerization activity of EF-Tuchl is also sensitive to the antibiotic kirromycin. In the presence of kirromycin, the apparent dissociation constant for the EF-Tuchl X GTP complex is lowered 10-fold. The interaction of aminoacyl-tRNA with EF-Tuchl was investigated by examining the ability of EF-Tuchl to prevent the spontaneous hydrolysis of Phe-tRNA and by gel filtration chromatography. The binding of aminoacyl-tRNA to EF-Tuchl occurs only in the presence of GTP indicating the formation of the ternary complex EF-Tuchl X GTP X Phe-tRNA. The effect of kirromycin on the interaction was also investigated. In the presence of kirromycin, no interaction between EF-Tuchl and Phe-tRNA is observed, even in the presence of GTP.