The demonstration of ribosome-bound penicillinase in [formula omitted

Abstract

Escherichia coli native aspartate transcarbamylase structure results from the zinc-dependent association of trimers of catalytic chains and dimers of regulatory chains. Thus it has been chosen as a model system to study the ultimate steps of protein biosynthesis which follow the establishment of the primary sequence of amino acids. It was reported that the biosynthesis of the two kinds of polypeptides is sequential in the order: catalytic chain then regulatory chain (Perbal & Hervé, 1972). It is now shown that their biosynthesis is correlated in the sense that the production of the regulatory chain is dependent on the biosynthesis of the catalytic chain. However an independent 5% residual biosynthesis of regulatory subunits is observed. Taken together, these results strongly suggest that the genes coding for the catalytic and regulatory chains are part of the same operon in the order of catalytic chain gene then regulatory chain gene.The utilization of preformed regulatory subunits in vivo for subsequent biosynthesis of complete enzyme, the absence of ribosome-bound aspartate transcarbamylase activity during the biosynthesis and complementation experiments between pyrB mutants, indicate that the association of chains and subunits does not occur on the polysomes but after release into the cytoplasm.