19.1.6—Marker genes for gene expression

A marker gene is transferred with a desired gene so that cells containing recombinant DNA can be selected or identified. The marker provides evidence about which cells received the construct; it is not the same DNA element as the promoter that starts transcription.

Selectable marker — gives transformed cells a survival or growth difference under the selection condition, allowing likely recombinant cells to be selected. Antibiotic-resistance markers can do this, but their possible spread to pathogenic bacteria may reduce antibiotic effectiveness.
Reporter marker — produces a visible or measurable signal, such as GFP fluorescence under ultraviolet light, allowing altered cells or marker expression to be located. Fluorescent markers avoid selecting with antibiotic-containing growth media and avoid the specific resistance-transfer concern.

1. Transfer the desired gene together with the marker construct.
2. Allow the host cells to grow under the relevant selection condition or use the reporter readout.
3. Retain or identify cells that show the marker phenotype or signal.
4. Interpret the result cautiously: marker detection supports successful construct transfer and/or marker expression, but does not by itself prove that the desired gene product is correctly folded, functional or at the expected level.

Promoter present and recognised → transcription of the linked gene can begin. Marker present and read out → cells carrying or expressing the construct can be found. Thus promoter and marker work at different checkpoints in the same construct: one supports expression, the other supports selection or identification.

A marker signal is not automatic proof of complete desired-product expression or function. Marker choice also has consequences: antibiotic-resistance markers can spread by genetic transfer, whereas fluorescent reporter markers can be detected directly with ultraviolet light. PCR, gel electrophoresis, microarrays and bioinformatics remain separate cards.