Use of radioactive phosphorus (³²P) in cell studies: after growing an Escherichia coli culture for 48 hours in medium containing radioactive phosphate, where would you expect the label to be found?

Introduction / Context:
Radioisotopic tracers are widely used to follow the incorporation of atoms into cellular macromolecules. Phosphorus is a core component of many biological molecules, so ³²P labeling reveals biosynthetic destinations in living cells.

Given Data / Assumptions:

• The medium supplies inorganic phosphate that cells assimilate.
• E. coli will be actively growing and dividing over 48 hours.
• Cellular macromolecules synthesized during growth will incorporate phosphate groups where chemically appropriate.

Concept / Approach:
Phosphate is incorporated into nucleic acids (DNA and RNA backbones as phosphodiester linkages) and phospholipids (phosphatidyl headgroups). Proteins (enzymes) generally lack phosphorus in their backbone, but many bacterial proteins undergo phosphorylation (post-translational modification) or bind tightly to phosphorylated metabolites, and some enzymes form covalent phospho-enzyme intermediates during catalysis. Over extended growth, detectable ³²P becomes associated with multiple cellular fractions.

Step-by-Step Solution:

Verification / Alternative check:
Fractionation experiments show strong ³²P signals in nucleic acids and membranes; phosphoproteomics and autoradiography detect ³²P in proteins after kinase-mediated labeling.

Why Other Options Are Wrong:
Picking only one class (RNA or phospholipids) ignores other major phosphate pools. Selecting only enzymes or only one macromolecule underestimates cellular phosphate distribution during growth.

Common Pitfalls:
Assuming proteins never contain phosphorus. While peptide backbones do not, phosphorylation and covalent intermediates can carry ³²P on proteins in vivo.

Final Answer:
all of these