Incorporation of certain amino acid analogs into polypeptides produced by cells which do not ordinarily provide polypeptides containing such amino acid analogs is accomplished by subjecting the cells to growth media containing such amino acid analogs. The degree of incorporation can be regulated by adjusting the concentration of amino acid analogs in the media and/or by adjusting osmolality of the media. Such incorporation allows the chemical and physical characteristics of polypeptides to be altered and studied. In addition, nucleic acid and corresponding proteins including a domain from a physiologically active peptide and a domain from an extracellular matrix protein which is capable of providing a self-aggregate are provided. Human extracellular matrix proteins capable of providing a self-aggregate collagen are provided which are produced by prokaryotic cells. Preferred codon usage is employed to produce extracellular matrix proteins in prokaryotics.