PROTEIN SYNTHESIS IN AGING AND REJUVENATED SOYBEAN COTYLEDONS
Abstract
Cotyledons of soybeans (Glycine max L., var. Wayne) were used as a model system to study quantitative changes in protein synthesis during aging and rejuvenation. Chlorophyll content increased from the time of light exposure to 8 days after germination; thereafter the content declined. DNA, RNA, protein, and polyribosomes declined rapidly after 4 days. After 20-day-old cotyledons were "rejuvenated" by epicotyl removal, the above components increased to between 12- and 16-day-old levels. About 47% of the high molecular weight polyribosomes were lost between days 4 and 8, but losses were slight during senescence. Rejuvenated cotyledons increased in high molecular weight polyribosomes, but they also had the highest percentage of monoribosomes. The translational capacity of polyribosomes in the wheat germ system declined rapidly from 4 to 12 days and slowly thereafter. Rejuvenation caused little improvement over senescent activity. Although young and rejuvenated polyribosomes produced high molecular weight proteins, few were produced by aging polyribosomes. This was not due to incomplete translation since ribosome runoff was nearly complete in all ages. Coextraction of 4- and 24-day-old cotyledons greatly reduced the translational activity of the 4-day-old polyribosomes but did not affect product size. Therefore, low activity with age was due to inactivation during extraction or to diffusible inhibitors associated with aging ribosomes. The content of poly(A)mRNA per cotyledon declined during aging and was increased by rejuvenation. Surprisingly, 4-day-old mRNA had the lowest translational activity in the wheat germ system, while aging and rejuvenated mRNAs were equally active. Translation products were compared on 2-dimensional gels. None were totally lost or newly synthesized during aging or rejuvenation. Of 470 products detected, 133 increased in abundance with age and 74 decreased. Rejuvenation caused a reversion to approximately 4-day old levels in 36 of the diminshed proteins and 40 of the increased proteins. This trend was also found in in vivo-labeled proteins. Many electrophoretic bands which either increased or decreased with age were partially restored to 4-day-old levels by rejuvenation. The amount of {('35)S} methionine incorporation in vivo declined with age but was restored to 4-day-old levels within 2 days of epicotyl excision.
Degree
Ph.D.
Subject Area
Botany
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