STRUCTURAL DEVIATIONS IN A BOVINE LOW EXPRESSION LYSOZYME-ENCODING GENE ACTIVE IN TISSUES OTHER THAN STOMACH

Lysozyme-encoding genes (Lys) constitute a gene-family in ruminants. W hile several of these genes are highly expressed in stomach (sLys), fe w other copies are weakly expressed in other tissues, notably in polym orphnuclear granulocytes and macrophages (mLys). Searching an understa nding for these grossly different levels of expression, we isolated th e bovine variant of the gene being expressed in granulocytes and chara cterized it by sequencing, together with its promoter. Spanning about 9 kb of genomic DNA, the gene is found to be segmented into four exons , in common with all other Lys, as known from vertebrates. Sequence ho mologies between all bovine sLys-variants exceeds 70% over much of the entire coding sequence and promoter region. This indicates (i) that b ovine lysozymes expressed either in stomach or granulocyte originate f rom a common ancestral gene and (ii) also excludes the possibility tha t the observed weak expression of the mLys gene is due to major struct ural rearrangements within the promoter segment. However, primer exten sion analysis based on RNA isolated from kidney locates the transcript ion startpoint (tsp of that gene) 44 nt further upstream than observed in both, bovine stomach lysozyme RNA or any of the homologous genes i n mice and man. The observed weak expression of this bovine mLys gene appears to be a consequence of both the presence of an extra ATG codon in the extended 5'-UTR, and a severe down mutation of the ancestral T ATA-box which is only partially compensated for by the presence of ano ther mutation further upstream resulting in a weak substitute promoter sequence.