GENETIC-BASIS FOR TISSUE ISOZYMES OF GLUTAMINE-SYNTHETASE IN ELASMOBRANCHS

Tissue-specific isozymes of glutamine synthetase are present in elasmo branchs. A larger isozyme occurs in tissues in which the enzyme is loc alized in mitochondria (liver, kidney) whereas a smaller form occurs i n tissues in which it is cytosolic (brain, spleen, etc.). The nucleoti de sequence of spiny dogfish shark (Squalus acanthias) liver glutamine synthetase mRNA, derived from its cDNA, shows there are two in-frame initiation codons (AUG) at the N-terminus which will account for the s ize differences between the two isozymes. Initiation at the up-stream and down-stream sites would yield peptides of 45,406 and 41,869 mol. w ts. representing the precursor of the mitochondrial isozyme and the cy tosolic isozyme, respectively. The additional N-terminal 29 amino acid s present in the mitochondrial isozyme precursor contains two putative cleavage sites based on the Arg-X-(Phe,Ile,Leu) motif. The predicted two-step processing would remove 14 of the 29 N-terminal amino acids. These 14 amino acids can be predicted to form a very strong amphipathi c mitochondrial targeting signal. Their removal would yield a mature p eptide of 43,680 mol. wt. The calculated mol. wts. based on the derive d amino acid sequence are therefore in good agreement with previous es timates of an approximately 1.5-2-kDa difference between the M(r)s of the mitochondrial and cytosolic isozymes. A model for the evolution of the mitochondrial targeting of glutamine synthetase in vertebrates is proposed.