Dephosphorylation methylation and cloning strain choice

Phosphatase, a dam/dcm strain, and a special cloning host answer three different failures. You are choosing the one that matches the evidence in front of you. Stacking them "to be safe" hides which problem was real and can create a new one, including a ligation with no phosphates left. This is a selection guide for that choice. It is not a list of banned enzymes and not a kit insert.

The join those choices feed is described in restriction ligation and Gibson assembly. The path to a colony is in plasmid cloning from insert to colony.

Three mechanisms that do not substitute for each other

Ordinary laboratory E. coli methylates DNA. Dam methylation marks the adenine in GATC. Dcm methylation marks a cytosine in the CCWGG sequence. If a restriction site overlaps one of those marks, and the enzyme card says the overlap blocks cutting, plasmid from a Dam-positive, Dcm-positive strain will not open at that site. The card is the authority. This page will not invent a list of blocked enzymes. Read the sensitivity line for the enzyme you have, then decide.

A dam/dcm-deficient strain does not add those marks, so a plasmid passaged through it can be cut when the card predicted a block. A PCR product is also unmethylated, because a polymerase reaction does not copy the methyl group. Either workaround is enough when the card is the reason. Neither one removes phosphates, and neither one makes a sensitive cell tolerate a ccdB cassette.

Phosphatase is an enzyme class that strips 5-prime phosphates. In a cloning ligation it is for the vector, and only when self-ligation is the problem you are trying to reduce: a single cut, blunt ends, or compatible ends that can close without an insert. The insert must keep its phosphates, from the restriction cut or from phosphorylated primers. Dephosphorylating both partners leaves ligase nothing to seal. Cleanup after the phosphatase matters so the activity does not travel into the ligation and nibble the insert. Follow the phosphatase sheet. This treatment does not demethylate DNA and does not change the strain.

The strain itself is a third lever, and it splits. A strain for a ccdB parent is the one the vector note names as able to maintain that cassette. Use it to propagate the parent. Use a sensitive strain when you need unrecombined parents to die. A strain for an unstable or repetitive insert is a different genotype, often reduced in homologous recombination, chosen because rearrangements were the evidence. Read the genotype string. Do not assume a dam/dcm strain is also the recombination-deficient strain, or that either one is the ccdB maintenance strain, unless the line in front of you says so.

Match one row, then stop adding fixes

Write the symptom in a sentence before you thaw cells or open phosphatase. A vector-only plate as full as the real plate is self-ligation or leftover uncut vector. A digest that will not cut a site the card calls methylation-sensitive, on DNA from an ordinary strain, is a methylation problem. A parent plasmid that kills a sensitive strain and grows in the maintenance strain is a cassette problem. A repetitive insert that rearranges between colony screening and the next prep is an instability problem. If you cannot write the sentence, you do not yet pick a fix.

Change one variable. Phosphatase the vector and keep the same strain. Or move the plasmid into the deficient strain and repeat the digest with no phosphatase involved. Or switch only the host for the cassette. Keep the intermediate DNA so a later failure can be walked back. Two changes in one afternoon feel efficient and produce a result you cannot assign.

Colony screening still comes after the choice. A phosphatase does not set insert orientation. Directional ends or a diagnostic cut do that. A new strain does not prove the junction. Sequence the clone you will keep, using the Sanger sequencing enquiry reference as the way to frame the read.

Record the DNA's last host on the tube. A plasmid prepped from a dam/dcm-deficient strain is the one you may cut at a blocked site. The same sequence prepped later from an ordinary strain can be methylated again and refuse that enzyme. If someone repeats your digest from a "fresh miniprep" in the usual host, the site can look blocked for a second time. The strain note prevents that false return to the enzyme card.

What goes wrong when the fixes are piled up

Phosphatase on a vector that was cut with two incompatible enzymes is often unnecessary. The ends already refuse to match each other. Adding phosphatase is harmless only if the insert still has phosphates and you clean the enzyme away. Adding it because the tube was open is how someone next treats the insert too.

A dam/dcm strain used for every miniprep "so sites always cut" makes that genotype your hidden default. Some preparations and some downstream methyl-sensitive applications care. When the card does not mention a block, stay with the ordinary strain and look for the real digest failure: inactive enzyme, the wrong buffer, a site that is not in this version of the plasmid, or star activity. The Addgene restriction digest protocol is a public note on the digest itself.

A ccdB parent transformed into a deficient strain that is still sensitive will not yield the parent. You will call the strain "bad" and order another. Check the cassette before you blame Dam. The maintenance strain and the methylation strain are different purchases unless one genotype line honestly includes both, and even then you use the cassette rule when you plate.

An unstable insert passaged through a strain that recombines readily will delete whether or not you dephosphorylated the vector. Phosphatase never stabilised a repeat. Choose the host for the rearrangement, then screen several colonies for the full length.

Safety

These are cloning-strain and enzyme choices for research constructs. They are not an approval to clone a hazardous insert, and they are not a preparation of any counterselection product. Follow the enzyme safety data and your institutional waste rules. A strain that maintains a cassette other strains die from is still a living recombinant host. Label it. The committee that covers your bench decides containment, not the genotype nickname.

Write the card line into the request

When you ask for a dam/dcm-deficient strain, quote the enzyme-card sentence that forced it, including the site you need to cut. When you ask for a maintenance strain, name the cassette and say it is for the parent only. When you ask for a strain for unstable inserts, name the repeat problem you already saw. Ordering all three in one note, with phosphatase "as well," stacks the variables this page is trying to separate. The molecular biology catalogue can be asked for an enzyme class or a strain class after that sentence exists.

If you would rather remove the blocked site from a designed sequence than passage the plasmid, that is a specification, not a strain. The custom gene synthesis enquiry reference is only the prompt for such a file. Put the single fix you chose on the quote request. Ask whether a quotation is possible. Send the symptom sentence with it.