A Coordinated Decline in the Synthesis of Subunits of Ribulosebisphosphate Carboxylase in Aging Wheat Leaves. II. Abundance of Messenger RNA

Abstract

RNA prepared from second leaves of wheat seedlings was used to prime in vitro synthesis of polypeptides in wheat germ and E. coli cell-free extracts. A major polypeptide synthesized in the wheat germ system had a molecular weight of c. 21 000 and was specifically precipitated with antibody to the small subunit of wheat ribulosebisphosphate carboxylase (EC 4.1.1.39). On these bases the polypeptide was identified as the precursor of the carboxylase small subunit. The major polypeptide synthesized in vitro in the E. coli system, primed either with total leaf RNA or with chloroplast RNA, had a molecular weight of c. 52 000 and was identified as the large subunit of the carboxylase enzyme by partial proteolytic digestion. The products synthesized in vitro in wheat germ or E. coli systems primed with RNA from different ages of leaves were compared. Translatable messenger RNAs for the small and large subunits of the carboxylase enzyme decreased in abundance relative to most of the other messenger RNAs in preparations from leaves of increasing age. The decreasing abundances of the translatable messenger RNAs were coordinated and closely resembled the declines in synthesis of the subunits observed in vivo [Brady, C. J. (1981). Aust. J. Plant Physiol., 8, 591-602]. The messenger RNA for the small subunit was shown to have a 7-methylguanosine (cap) sequence at its 5' terminal; however, loss or modification of the cap sequence was not involved in regulating availability of translatable messenger RNA.