Promoters tags and reading frames

A promoter, a tag and a reading frame are three separate claims on a plasmid, and they are easy to nod through because the map draws them as neat arrows. Before you transform, you can check all three on the sequence. After you transform, a resistant colony will not tell you that you were right. This overview is the check. The cloning path around it is in plasmid cloning from insert to colony.

Backbones and strains are sourcing questions for the molecular biology catalogue. The sequence file is the object you are checking.

Name the promoter by the polymerase that reads it

A T7 promoter is recognised by T7 RNA polymerase, not by the ordinary bacterial polymerase. The polymerase has to come from somewhere else: commonly a lambda DE3 lysogen in which T7 gene 1 sits under lacUV5 control, or a second plasmid that supplies it. Without that source, a perfect T7 construct is a silent high-copy plasmid in a cloning strain. You may build the clone in a strain that lacks the polymerase, so a toxic insert stays quiet, and move it into the expression host later. Say which strain is which in the plan. A colony on ampicillin in a non-DE3 strain confirms the marker. It does not confirm transcription.

Lac and its stronger relatives, such as tac and trc, are a different claim. The host RNA polymerase reads them. Lac repressor, from a lacI or lacIq gene on the plasmid or the chromosome, keeps them quiet until an inducer is added. IPTG appears in both stories, which is why people merge them. On a lac vector, IPTG acts at the promoter in front of your gene. On a typical T7 vector, IPTG acts on the gene that makes T7 RNA polymerase, and that polymerase then finds the T7 promoter. Substituting one vector for the other because both "are IPTG inducible" gives you the wrong polymerase requirement. Other bacterial promoters, constitutive or tightly regulated, need their own repressor or activator story. Read the sentence on the map that names the promoter, then write the host genotype next to it.

Addgene's molecular biology reference is a public glossary for these promoter classes. It is not a genotype for your freezer stock. protocols.io shows how groups record host and inducer together. Follow the strain notes you have.

The ribosome binding site and the start you mean

In a bacterial host the ribosome binding site, the Shine-Dalgarno sequence, sits a short distance upstream of the AUG that should initiate. The spacing is part of the design. A tag cassette usually brings its own start and its own ribosome binding site. If you then clone an insert that still has its native start, the message can initiate at two places and you purify a mixture, or the upstream start dominates and the insert start is just more codons. Decide. For an N-terminal tag, the tag's ATG is usually the start and the insert's ATG is removed or becomes residue two on purpose. For an untagged insert, the vector's ribosome binding site must sit at a working distance from the insert ATG, not from a leftover piece of polylinker that contains an earlier ATG.

Look up the protein sequence you think you are cloning in UniProt and, where a nucleotide record exists, in NCBI GenBank. Use those accessions as a sequence reference for the start and the stop of the natural chain. They are not a claim that a tagged fusion is a catalogue product, and they are not evidence your junction is intact.

Count the frame across the fusion junction

Codons are three bases. From the ATG you chose, mark every third base through the tag, the linker, and the scar the cloning method left behind. A restriction site between tag and insert contributes its bases to that count. An overlap from a Gibson-style join contributes whatever bases you left in the overlap. If the count arrives at the first codon of the open reading frame, the frame is intact. If you are one base long or one base short, every downstream amino acid changes. That is a frameshift. Three extra bases insert one amino acid and preserve the frame, which may still alter a cleavage site or an activity, but it is not a frameshift.

The map label "6xHis" is a feature track. It stays in frame only when this count says so. Software will draw the track on a sequence that slipped by a base during primer design. Translate the fusion in the frame you counted and read the amino acids across the junction with your eyes. A run of histidines that turns into a stop or into unrelated residues is the finding. Colony screening that reports insert size will not see it. Insert orientation matters too: a backward insert has no sensible frame relative to this promoter, even if each half was translated correctly in a notebook.

Stops belong in the same count. An N-terminal tag fusion must not contain a stop between the tag and the protein, or the ribosome finishes at the tag and the insert never becomes polypeptide. A C-terminal tag requires the insert's natural stop to be omitted so translation continues into the tag, and then a stop in frame after the tag. A vector that already hides a stop in the multiple cloning site will end the protein there if that stop is in frame, which is what you want only when you meant an untagged product. Write down which stop is supposed to win.

A staged check before the cells come out

Print or open the annotated sequence, not a cartoon that hides the bases. Circle the promoter and write the polymerase beside it. Circle the ribosome binding site and the ATG. Translate in threes to the junction and past it into the insert for at least twenty codons. Find the stop that ends this translation and confirm it is the stop you wanted, after a C-terminal tag if there is one. Then, and only then, transform. If the translation breaks, fix the primer or the synthetic file. A frameshift is cheaper to edit before it becomes a colony with a name.

If the insert arrived by PCR, remember that a correct frame on the designed primer does not protect you from a polymerase error later in the gene. The frame check on the map is about the design. Confirmation of the clone still means reading the insert. Colony screening and orientation tests come after this paper check. They do not replace it.

Branch if the promoter and the host disagree. Do not induce a T7 plasmid in a strain you grabbed because it was competent. Move the finished plasmid into the polymerase-bearing host, or change the promoter on the design. Branch if two ATGs remain. Delete one in the file before you order the primer. Branch if the stop is early. You will not purify a C-terminal tag that is never translated.

When the protein does not match the drawing

A product that is far too short is often an early stop from a frameshift or from a mutation, not a mysterious protease, until you have translated the clone. A product that purifies without the tag is often a start at the insert ATG, downstream of a tag that is present on a longer species you did not notice. A product that is absent, with a perfect frame, sends you back to the promoter and the polymerase source. Order the questions that way so a frame error is not hidden inside an expression-condition project.

Safety

Expression of an insert can be the hazardous step, especially if the protein is toxic or comes from a pathogen. Your institution decides whether that induction is allowed. A correct frame does not lower a biosafety level. This overview is research education. It is not a method for producing a therapeutic protein and not a diagnostic claim about any gene.

Two lines on the enquiry, not one

When you ask for a backbone, a strain or a synthetic fusion, write the promoter class and the polymerase source as separate lines. A single line that says "expression vector, IPTG" invites a lac plasmid where you needed T7, or the reverse. Name the tag, the end it occupies, and that the junction must stay in frame. In a shared building the file should carry that sentence even if the conversation is brief. The frame will not be rescued by a colder room or a faster courier.

What to send

Attach the annotated sequence, the UniProt accession you used as a sequence reference, the promoter name, the host genotype, and the translation across the junction. A designed fusion can be discussed through the custom gene synthesis enquiry reference. A read that proves the junction bases can be framed with the Sanger sequencing enquiry reference. Those pages are enquiry references. They do not mean EVRINTH synthesises the gene or runs sequencing, and an accession is not a product they are selling. Put the requirement in the quote request and ask whether a quotation is possible.