Escherichia coli (E. coli) is a bacterium found in the environment and the intestines of people and animals. Its simple structure and rapid growth make it a model organism for molecular biology research, particularly for understanding how cells respond to DNA damage. Ultraviolet (UV) light is a form of radiation with enough energy to cause chemical changes in biological molecules. The primary target of this energy in a bacterial cell is its DNA, leading to events that determine the cell’s fate.
The Formation of Pyrimidine Dimers
When UV radiation, specifically UV-C and UV-B, penetrates an E. coli cell, the energy is absorbed by the DNA molecule. This drives a chemical reaction between adjacent pyrimidine bases on the same DNA strand, such as cytosine (C) and thymine (T). The UV energy facilitates the formation of covalent bonds between these neighboring bases, creating a lesion known as a pyrimidine dimer.
The most frequent types of these lesions are cyclobutane pyrimidine dimers (CPDs) and (6-4) pyrimidine-pyrimidone photoproducts. The formation of a thymine-thymine dimer is a common outcome. This new covalent linkage creates a physical distortion in the DNA’s double helix structure. The normally smooth DNA develops a bulge or a kink at the site of the dimer, altering the helix’s spacing.
This structural change is the foundational damage from which all other effects of UV exposure originate. The severity of the damage, meaning the number of pyrimidine dimers formed, is directly correlated with the dose of UV radiation the cell receives. The formation of these dimers is a purely photochemical process, a direct consequence of the DNA molecule’s interaction with high-energy photons, fundamentally altering the genetic blueprint of the organism.
Disruption of Key Cellular Processes
The physical kink in the DNA helix caused by a pyrimidine dimer acts as a roadblock for the molecular machinery that interacts with the genetic code. Two processes are immediately affected: DNA replication and transcription. These functions are central to the life of the E. coli cell, and their interruption has paralyzing consequences.
During DNA replication, an enzyme called DNA polymerase moves along the DNA strand to synthesize a new, complementary strand. When the polymerase encounters a pyrimidine dimer, its progress is blocked. The structural distortion prevents the enzyme from correctly identifying the bases, causing the replication fork to stall. This means the cell can no longer duplicate its chromosome for cell division.
A similar problem occurs during transcription, where the enzyme RNA polymerase copies DNA into RNA. This enzyme is also unable to move past the distorted structure of a pyrimidine dimer. The blockage of transcription prevents the synthesis of messenger RNA (mRNA), which carries instructions for building proteins. Without new proteins, the cell cannot perform metabolic functions or produce enzymes for its own maintenance.
E. coli’s DNA Repair Mechanisms
E. coli has evolved distinct repair systems to find and correct pyrimidine dimers, restoring the DNA molecule’s integrity. The cell’s first line of defense is a direct repair pathway known as photoreactivation, or light repair. This process relies on an enzyme called photolyase, which recognizes and binds to the pyrimidine dimers.
The photolyase enzyme contains cofactors that absorb energy from visible light, particularly in the blue part of the spectrum. This light energy is then used to break the covalent bonds forming the dimer. This directly reverses the damage and restores the two pyrimidines to their original states.
When visible light is not available, E. coli employs a more complex mechanism called Nucleotide Excision Repair (NER), or dark repair. This multi-protein system excises the damaged portion of the DNA strand. The process is initiated by proteins that scan the DNA for distortions. Upon finding a lesion, other proteins make two cuts in the sugar-phosphate backbone on either side of the damage, and a helicase enzyme removes the segment containing the dimer. DNA polymerase I then synthesizes a new, correct segment using the undamaged opposite strand as a template, and DNA ligase seals the final gap.
If the DNA damage is widespread and overwhelms these primary repair systems, the cell can trigger a last-resort response known as the SOS response. This system is activated by the accumulation of single-stranded DNA from stalled replication forks. The SOS response activates more than 40 genes, many involved in DNA repair. While it helps the cell survive, some activated DNA polymerases are “translesion” polymerases, which can replicate past the dimer but are error-prone, often inserting incorrect bases.
Cellular Fates After UV Exposure
The outcome for an E. coli cell following UV exposure is determined by a balance between the extent of the DNA damage and the efficiency of its repair. If the UV dose is low, resulting in a manageable number of pyrimidine dimers, the cell’s repair systems are effective. Photoreactivation and Nucleotide Excision Repair can accurately remove the dimers, restoring the DNA. The cell then resumes normal functions and continues to grow and divide.
Conversely, a high dose of UV radiation can create an overwhelming number of pyrimidine dimers, saturating the cell’s repair capacity. When the damage is too extensive to be fixed in a timely manner, the persistent stalling of DNA replication and transcription becomes fatal. The cell is unable to produce essential proteins or reproduce its genetic material, leading to a complete shutdown of cellular function and ultimately, cell death. This bactericidal effect is the principle that underlies the use of UV-C light as a powerful tool for sterilization and disinfection in medical and industrial settings.
A third possibility exists for cells that experience heavy but not immediately lethal damage. Under such stress, the SOS response may be activated to bypass the damage and allow replication to proceed. While this prevents immediate cell death, it comes at the cost of genetic fidelity. The process often inserts incorrect bases opposite the dimer, resulting in the cell surviving with permanent changes, or mutations, fixed in its DNA.