Cook macaroni in boiling salted water until tender, yet firm. Drain. Melt butter in saucepan; blend in flour. Add soy sauce and 1 ¾ cups water all at once; stir until well blended. Simmer until sauce thickens, stirring constantly. Remove from heat and blend in pepper and ¾ of cheese. Combine tuna, macaroni and green pepper in ungreased 1 ½ quart baking dish. Stir in cheese sauce. Sprinkle remaining cheese over top. Bake in 425°F oven until top is golden brown, about 15 minutes.