Multiple Genes for Lysozyme in Birds: Studies on Black Swan Egg White Lysozymes

Abstract

In contrast to other bird species that have been studied, not one but two distinct lysozymes have been isolated from the egg white of the black swan (Cygnus atratus). One is similar in molecular weight, NH2-terminal amino acid sequence, amino acid composition, specific activity, and immunological cross-reactivity to the lysozymes found in the egg white of the chicken, duck, and many other birds. The other black swan lysozyme shows no resemblance to these “chick-type” enzymes by the above criteria. It is very similar to the lysozyme found until now only in the egg white of the Embden goose. This black swan lysozyme and Embden goose lysozyme have a molecular weight of about 20,500 and are identical in the sequence of their first 20 NH2-terminal amino acids and in immunological cross-reactivity. Amino acid composition data indicate that these two “goose-type” lysozymes differ in sequence by a minimum of four amino acid substitutions. The differences in the structures of the chick-type and goose-type lysozymes suggest that they are products of two distinct genetic loci. Some species contain only chick-type lysozymes in their egg white (e.g. chicken, duck), others only goose-type lysozymes (Embden goose), while others contain both types of enzyme (black swan). In the light of other evidence, this suggests that the genetic loci coding for both enzymes are present in all species and that the presence of either or both lysozymes in egg white is controlled by a species-specific regulatory mechanism.