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Magnesium starvation of Aerobacter aerogenes. 3. Protein metabolism
Abstract:
The metabolism of the ribosomal and soluble protein components of Aerobacter aerogenes was examined during its incubation in a Mg(++)-deficient medium. Bacteria were exposed to leucine-H(3) during the exponential growth period preceding Mg(++) starvation, and extracts were prepared after intervals of starvation and were centrifuged through gradients of sucrose to separate ribosomal from soluble proteins. Ribosomal proteins synthesized during the preceding exponential growth were slowly lost from the ribosomes; after 8 hr of starvation, few, if any, sedimented with ribosomes. Losses of total protein, together with the known rate of ribosome decay during Mg(++) starvation, suggested that these ribosomal proteins are ultimately degraded to acid-soluble products and account for all protein lost by the starving cells. These conclusions were supported by studies of Mg(++) starvation in a uracil-requiring strain of A. aerogenes: during uracil starvation a smaller fraction of the proteins synthesized were ribosomal, and the fraction of protein which subsequently decayed during Mg(++) starvation was correspondingly less. During recovery from Mg(++) starvation, proteins, lost from disintegrated ribosomes, were not detectably reutilized into new particles even before their degradation to acid-soluble products was complete. Synthesis of soluble proteins continued for more than 24 hr of starvation at a rate per milliliter close to 45% of the instantaneous rate per milliliter of the exponentially growing bacteria at the time Mg(++) was removed. This value agreed with that found previously for synthetic rates of deoxyribonucleic acid, transfer ribonucleic acid, and ribosomal ribonucleic acid during starvation relative to rates during exponential growth.
Insights
During magnesium starvation, Aerobacter aerogenes degrades ribosomal proteins into soluble products. These lost proteins are not reused, and soluble protein synthesis continues at a reduced rate.
Area of Science:
- Microbiology
- Molecular Biology
- Bacterial Metabolism
Background:
- Bacterial cells maintain essential functions through protein synthesis and degradation.
- Ribosomes are crucial for protein synthesis and are composed of ribosomal proteins and RNA.
- Magnesium is an essential cofactor for numerous cellular processes, including ribosome function.
Purpose of the Study:
- To investigate the metabolic fate of ribosomal and soluble proteins in Aerobacter aerogenes during magnesium deficiency.
- To determine if ribosomal proteins synthesized before starvation are degraded or reutilized.
- To quantify the synthesis rate of soluble proteins during magnesium starvation.
Main Methods:
- Aerobacter aerogenes cultures were grown and then subjected to magnesium-deficient conditions.
- Radioactive leucine (leucine-H3) was used to label proteins during exponential growth.
- Cell extracts were separated into ribosomal and soluble protein fractions using sucrose gradient centrifugation.
- Protein degradation and synthesis rates were analyzed during starvation and recovery periods.
Main Results:
- Ribosomal proteins synthesized during exponential growth were progressively lost from ribosomes during magnesium starvation.
- The loss of ribosomal proteins accounted for the total protein loss observed in starving cells, suggesting degradation to acid-soluble products.
- Synthesis of soluble proteins continued during starvation, albeit at a reduced rate (approximately 45% of the growth rate).
- There was no detectable re-utilization of proteins from disintegrated ribosomes during recovery from starvation.
Conclusions:
- Magnesium starvation triggers the degradation of existing ribosomal proteins in Aerobacter aerogenes.
- Degraded ribosomal proteins are broken down into acid-soluble products and are not reutilized for new protein synthesis.
- Soluble protein synthesis persists during magnesium starvation, indicating a differential regulation of protein metabolism.