Differentially charged analogues of block copolymers containing repeating sequences from silk (GAGAGS) and elastin (GVGVP) were synthesized using genetic engineering techniques by replacing a valine residue with glutamic acid. The sensitivity to pH and temperature was examined at various polymer concentrations, ionic strengths, and polymer lengths. The polymers transitioned from soluble to precipitate state over narrow temperature ranges. The transition temperature T-t (the temperature at which half-maximal spectrophotometric absorption was observed) increased with increasing pH up to pH 7.0 and leveled off above this value for the Glu-containing polymer (17E)(1t). T-t was independent of pli for the Val-containing polymer (17V)(1t). It decreased with increasing ionic strength, polymer concentration, and polymer length for both polymers. These results suggest that by substituting charged amino acids for neutral amino acids at strategic locations in the polymer backbone and by control of the length of silkelastin-like block copolymers using genetic engineering techniques, it is possible to precisely control sensitivity to pH, temperature, and ionic strength.