Does secA mediate coupling between secretion and translation in Escherichia coli?

Area of Science:

• Molecular Biology
• Cell Biology
• Microbiology

Background:

• The secA gene is crucial for protein secretion in Escherichia coli.
• Amber mutations in secA lead to decreased synthesis of secreted proteins like maltose-binding protein (MBP).
• Previous studies suggested a link between protein secretion and translation, involving signal sequences and cellular machinery.

Purpose of the Study:

• To re-evaluate the relationship between protein secretion and translation in E. coli secA mutants.
• To investigate the role of signal sequence mutations in overcoming secA-mediated synthesis inhibition.
• To determine the underlying cause of reduced MBP synthesis in secA(Am) strains.

Main Methods:

• Analysis of secA amber (secA(Am)) mutant strains of Escherichia coli.
• Introduction of signal sequence mutations into malE (MBP) and alkaline phosphatase genes.
• Assessment of protein synthesis levels and secretion defects under varying conditions.
• Investigation of cyclic AMP (cAMP) levels and their impact on gene transcription and secretion.

Main Results:

• Signal sequence mutations did not selectively rescue MBP or alkaline phosphatase synthesis in secA(Am) strains.
• Significant variability in synthesis inhibition was observed among closely related secA(Am) strains.
• Reduced MBP synthesis in secA(Am) strains was linked to cyclic AMP depletion and decreased malE gene transcription.
• Secretion defects in secA(Am) strains were independent of cyclic AMP levels.

Conclusions:

• The inhibition of MBP synthesis in secA(Am) strains is a secondary consequence of primary protein export defects.
• The observed effects are not due to a direct coupling of translation and secretion involving signal sequences.
• Cyclic AMP depletion contributes to reduced transcription of certain secreted protein genes but does not cause the core secretion defect.