| he conventional lysozyme c gene of mammals provides a model which demonstrates many aspects of gene evolution. The lysozyme gene has been recruited to new functions, evolved convergently in both sequence and function, changed its rate of evolution, and duplicated. Most mammals have a single lysozyme gene, which functions as a host defense enzyme. Advanced ruminants, such as cow and sheep, possess at least ten lysozyme genes, four of which are expressed in the stomach. The pig, a non-ruminant, diverged from the lineage leading to the ruminants about 55 million years ago. Pig lysozyme is expressed in the stomach, but is not adapted to function in the stomach environment. It appears that the pig lysozyme gene represents an intermediate stage in the evolution of a stomach lysozyme.;To gain more insight into the evolution of the stomach lysozyme gene, and to identify cis-regulatory elements that control tissue-specific gene expression, the pig lysozyme gene was isolated and characterized. Southern blot analysis confirmed that the pig has only a single lysozyme gene. Northern blot analysis and nuclease S1 mapping indicate that expression of the pig lysozyme, in both stomach and non-stomach tissues, is driven by the same promoter. Phylogenetic analysis demonstrated that the large number of amino acid substitutions that occurred during the evolution of the stomach lysozyme gene in the early ruminant were due to changes in selective constraints, and probably due to adaptive evolution. Transfection of part of the pig lysozyme promoter into monocyte cells suggests that the region between... |
