The enzyme glutamine synthetase I of Drosophila melanogaster is associated with a modified RNA

Abstract

Glutamine synthetase I (ZZZl-glutamate:ammonia ligase, ADP forming; EC 6.3.1.2) was purified from Drosophila melanogaster larvae. The complete enzyme has an apparent molecular weight of 380,000. The subunit of the active enzyme has an apparent molecular weight of 43,000 after sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. Routine preparations yield enzymes which have at least another polypeptide component of apparent molecular weight of 64,000. Several factors suggest that the 64,000-dalton polypeptide might be a transformation product of the 43,000-dalton subunit which occurs in association with enzyme inactivation. Distinct from its protein subunit, from pure glutamine synthetase I a material can be extracted which can be labeled with ³²P-labeled γ-ATP using polynucleotide kinase. After alkaline hydrolysis the majority of the radioactivity is recovered as 5′2′ and 5′3′ ribonucleotide diphosphates, and after venom phosphodiesterase digestion as 5′ ribonucleotide. We therefore conclude that the native glutamine synthetase I enzyme contains, or at least is reproducibly associated with, an RNA component. Several characteristics of the labeled material indicate that the RNA is small in size and is bound to polymer molecules different from RNA.