E. coli was engineered to express two different fluorescent proteins (GFP and
RFP) under identical promoters. The strain is engineered such that the two
proteins are expressed at a 1:1 ratio, giving the bacteria a ‘yellow’ colour. As the
culture grows further over time, which of the following scenarios is most likely?
Assuming that neither GFP nor RFP confers any kind of selection pressure.
GFP starts predominating over RFP, turning the cultures green
RFP starts predominating over GFP, turning the culture red
Individual bacterium in the culture starts expressing the two proteins
distinctly, providing a mosaic pattern
Robust regulatory mechanisms for transcription and translation ensure
the maintenance of 1:1 ratio, thus the culture will remain ‘yellow’.

The most likely scenario is that individual E. coli cells will show different GFP:RFP ratios due to stochastic (noisy) gene expression, so the population will display a mosaic of green‑biased, red‑biased and yellow cells rather than a perfectly uniform yellow or a stable shift to only GFP or only RFP.​

Correct option and core concept

Correct choice:
“Individual bacterium in the culture starts expressing the two proteins distinctly, providing a mosaic pattern.”

Even though both fluorescent proteins are placed under identical promoters and designed for a 1:1 expression ratio, transcription and translation in single cells are inherently noisy processes. This gene expression noise leads to cell‑to‑cell variability in protein levels, so some cells will be greener (GFP‑high), some redder (RFP‑high), and some approximately yellow.​

Why a mosaic pattern arises

• Gene expression in bacteria fluctuates randomly because promoter ON/OFF switching, transcription bursts and translation bursts are probabilistic events.​

• Even for identical promoters and ribosome binding sites, each copy of the gene experiences independent fluctuations, so GFP and RFP numbers per cell diverge over time.​

• Experiments tracking dual reporters (e.g., GFP and RFP under similar control sequences) consistently show broad distributions of expression and substantial cell‑to‑cell variability, rather than a sharp single ratio in every cell.​

At the macroscopic level, if you observe bulk culture fluorescence, the average emission may still approximate “yellow,” but imaging individual cells will reveal a heterogeneous mosaic pattern.​

Why the other options are unlikely

GFP predominates and culture turns green

This would require a systematic advantage for GFP over RFP such as:

• Higher promoter strength or translation efficiency for gfp.

• Greater protein stability or faster maturation of GFP.

• A fitness effect where GFP‑high cells grow faster.

Under the question’s assumptions, promoter and other control elements are identical and neither GFP nor RFP confers selective advantage, so there is no directional bias for GFP to dominate the entire population. Random noise alone does not drive a consistent, population‑wide shift toward one colour.​

RFP predominates and culture turns red

The same reasoning applies here. For the culture to become red overall, RFP expression or stability would have to be consistently favoured, or RFP‑high cells would need a growth advantage. Since the problem states there is no selection pressure associated with either protein and control is designed to be symmetric, a systematic drift to red is no more likely than a drift to green, and neither is expected as a stable outcome.​

Culture remains perfectly yellow (strict 1:1 in all cells)

The option stating “Robust regulatory mechanisms for transcription and translation ensure the maintenance of 1:1 ratio, thus the culture will remain ‘yellow’” is not realistic at single‑cell level.

• Bacterial transcription and translation are not deterministically locked to exact ratios; instead they exhibit measurable noise, typically quantified as a coefficient of variation of protein numbers per cell.​

• Even essential and tightly controlled promoters still show finite noise and cannot maintain an exact same copy number of GFP and RFP in every cell at all times.​

Therefore, while the average expression across millions of cells can approach a 1:1 ratio (and the culture may look yellow in bulk), individual cells will deviate from this ratio, producing the mosaic.

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SEO‑friendly introduction (using keyphrase)

Stochastic gene expression in E. coli GFP RFP systems explains why a strain engineered to express green and red fluorescent proteins at a nominal 1:1 ratio does not remain perfectly yellow over time. Even with identical promoters and no selective advantage for either fluorescent protein, random transcription and translation events generate cell‑to‑cell variability, producing a mosaic of green, red and yellow cells within the same culture.​