You will be presented with the full text of a biomedical research paper. Please write an abstract for this paper. Your response should include the abstract and no additional text.

Paper text:
the psesame - cre expression vector was constructed by inserting a cre - encoding fragment into psesame via avrii and nhei restriction sites using standard cloning methods .
psesame encodes a fusion protein consisting of a histidine - tag , tat - domain , nls sequence and cre , abbreviated htncre . for expression of htncre
the psesame - cre was transformed into tuner ( de3 ) placi and used to prepare a glycerol stock .
an over - night culture was inoculated using a pipet tip coated with transformed bacteria from the glycerol stock .
the over - night culture consisted of lb media supplemented with 0.5% glucose [ v / v ] and carbenicillin at a final concentration of 50 g / ml and was allowed to grow at 37c for 16 hours .
next day the densely grown over - night culture was used to inoculate the expression culture at a ratio of 1 to 40 and was put in an incubator at 37c .
expression culture consisted of tb media supplemented with 0.5% glucose [ v / v ] and ampicillin at a final concentration of 100 g / ml .
at an od595 of 1.5 the expression culture was induced with 0.5 mm iptg for 1 h. subsequently bacteria were collected by centrifugation at 5000 rpm for 10 minutes in a sla3000 rotor .
frozen bacteria pellets were resuspended in 10 ml lysis buffer per liter flask culture for 15 minutes at room temperature .
suspension was then incubated with 1 mg / ml lysozyme for additional 15 minutes while mixing at room temperature .
25 u / ml benzonase was added afterwards and incubated while mixing for 15 minutes at room temperature .
after sonification on ice for 1.5 min with 0.5 s pulses at 45% of the power , 1 ml cold tartaric salt buffer ( tsb ) per ml suspension was carefully added while mixing and incubated for 5 min on ice .
sds - page samples of soluble ( s ) and insoluble fractions ( i ) were taken .
the supernatant was transferred into fresh 50 ml falcon tubes and was then gently mixed for 1 h at 4c with 2 ml of 50% ni - nta slurry per liter of initial expression culture .
the suspension was packed into a gravity flow econopac column ( sds - page sample of flow - through fraction ( ft ) was taken ) and washed twice with 5 bed - volumes of washing buffer .
sds - page samples of both washing fractions ( w1 & w2 ) were collected .
htncre - containing fractions were eluted with 3 bed - volumes of elution buffer and sample of eluate fraction ( e ) for sds - page analysis was taken .
the protein solution was further concentrated by dialyzing against glycerol buffer twice . in all dialysis steps
this procedure resulted in a glycerol stock solution containing htncre at a usual concentration between 200 and 450 m , i.e. 1 liter of expression culture will result in ~12 mg of protein .
figure 1 : sds - page analysis of the samples collected during the purification process of cre recombinase .
although a part of the protein is insoluble the cre protein can be further enriched as seen in eluate and glycerol stock fractions .
l : lysate , i : insoluble , s : supernatant , ft : flow - through , w : washing , e : eluate , gs : gylcerol stock .
es cells carrying a conditional -galactosidase reporter construct were seeded as single cells using tryple express for dissociation of adherent cells .
after 4 to 6 hours the cells had re - attached and the medium was removed .
an appropriate amount of htncre protein ( corresponding to 10 m ) out of the glycerol stock was diluted into es medium and subsequently sterile filtrated ( 0.22 m )
. an appropriate amount of htncre protein ( corresponding to 10 m ) out of the glycerol stock was diluted into es medium and subsequently sterile filtrated ( 0.22 m ) . after protein transduction medium
after two days cells were washed with pbs and fixed with 4% paraformaldehyde ( pfa ) for 10 minutes .
two additional washing steps with pbs were executed before x - gal staining was performed .
fixed cells were covered with a layer of x - gal staining solution and incubated over night at 37c .   next day
x - gal staining solution was aspired and the cells were covered with a layer of pbs for microscopy analysis . 80 to 100% of recombined cells could be observed within the murine es cells judged by -galactosidase activity .
the psesame - cre expression vector was constructed by inserting a cre - encoding fragment into psesame via avrii and nhei restriction sites using standard cloning methods .
psesame encodes a fusion protein consisting of a histidine - tag , tat - domain , nls sequence and cre , abbreviated htncre . for expression of htncre
the psesame - cre was transformed into tuner ( de3 ) placi and used to prepare a glycerol stock .
an over - night culture was inoculated using a pipet tip coated with transformed bacteria from the glycerol stock .
the over - night culture consisted of lb media supplemented with 0.5% glucose [ v / v ] and carbenicillin at a final concentration of 50 g / ml and was allowed to grow at 37c for 16 hours .
next day the densely grown over - night culture was used to inoculate the expression culture at a ratio of 1 to 40 and was put in an incubator at 37c .
expression culture consisted of tb media supplemented with 0.5% glucose [ v / v ] and ampicillin at a final concentration of 100 g / ml .
at an od595 of 1.5 the expression culture was induced with 0.5 mm iptg for 1 h. subsequently bacteria were collected by centrifugation at 5000 rpm for 10 minutes in a sla3000 rotor .
frozen bacteria pellets were resuspended in 10 ml lysis buffer per liter flask culture for 15 minutes at room temperature .
suspension was then incubated with 1 mg / ml lysozyme for additional 15 minutes while mixing at room temperature .
25 u / ml benzonase was added afterwards and incubated while mixing for 15 minutes at room temperature .
after sonification on ice for 1.5 min with 0.5 s pulses at 45% of the power , 1 ml cold tartaric salt buffer ( tsb ) per ml suspension was carefully added while mixing and incubated for 5 min on ice .
sds - page samples of soluble ( s ) and insoluble fractions ( i ) were taken .
the supernatant was transferred into fresh 50 ml falcon tubes and was then gently mixed for 1 h at 4c with 2 ml of 50% ni - nta slurry per liter of initial expression culture .
the suspension was packed into a gravity flow econopac column ( sds - page sample of flow - through fraction ( ft ) was taken ) and washed twice with 5 bed - volumes of washing buffer .
sds - page samples of both washing fractions ( w1 & w2 ) were collected .
htncre - containing fractions were eluted with 3 bed - volumes of elution buffer and sample of eluate fraction ( e ) for sds - page analysis was taken .
the protein solution was further concentrated by dialyzing against glycerol buffer twice . in all dialysis steps
this procedure resulted in a glycerol stock solution containing htncre at a usual concentration between 200 and 450 m , i.e. 1 liter of expression culture will result in ~12 mg of protein .
figure 1 : sds - page analysis of the samples collected during the purification process of cre recombinase .
although a part of the protein is insoluble the cre protein can be further enriched as seen in eluate and glycerol stock fractions .
l : lysate , i : insoluble , s : supernatant , ft : flow - through , w : washing , e : eluate , gs : gylcerol stock .
es cells carrying a conditional -galactosidase reporter construct were seeded as single cells using tryple express for dissociation of adherent cells .
after 4 to 6 hours the cells had re - attached and the medium was removed .
an appropriate amount of htncre protein ( corresponding to 10 m ) out of the glycerol stock was diluted into es medium and subsequently sterile filtrated ( 0.22 m ) .
an appropriate amount of htncre protein ( corresponding to 10 m ) out of the glycerol stock was diluted into es medium and subsequently sterile filtrated ( 0.22 m ) . after protein transduction medium
after two days cells were washed with pbs and fixed with 4% paraformaldehyde ( pfa ) for 10 minutes .
two additional washing steps with pbs were executed before x - gal staining was performed .
fixed cells were covered with a layer of x - gal staining solution and incubated over night at 37c .
next day x - gal staining solution was aspired and the cells were covered with a layer of pbs for microscopy analysis . 80 to 100% of recombined cells could be observed within the murine es cells judged by -galactosidase activity .
during the purification process of the cre fusion protein it is important not to omit the addition of ice  cold tbs buffer prior to centrifugation .
if the eluate fraction appears to become turbid due to the high concentration of fusion protein additional elution buffer should be added until the solution has cleared again .
the application of 10 m of cre fusion protein typically results in a recombination efficiency of 80 to 100% . fetal calf serum ( fcs ) being a major component of es cell medium strongly inhibits protein transduction
when working in serum - free conditions less protein ( 0.5 - 2 m ) can be used to achieve similar recombination efficiencies . with the psesame vector system at hand one can apply the technique of protein transduction to other proteins including transcription factors such as oct4 and sox2 and scl / tal1 .
the protein transduction technique enables the direct delivery of biologically active material into mammalian cells [ for review see 1,2 ] . for this one can make use of the translocating ability of so - called cell penetrating peptides ( cpps ) , 
 also designated as protein transduction domains ( ptds ) . 
 the tat - cpp derived from the human immunodeficiency virus type 1 ( hiv-1 ) tat ( trans - activator of transcription ) protein has been widely used . 
 the positively charged tat promotes cell permeability thereby overcoming the barriers of the cellular membrane by endocytosis or / and direct membrane penetration2 . in combination with a nuclear localization signal ( nls ) 
 fusion proteins are able to enter the nucleus exhibiting functionality . 
 our video presentation demonstrates , as an exemplification for the engineering of cell - permeable proteins , the construction , production and application of a cell - permeable version of the dna - modifying enzyme cre.cre is a site - specific recombinase that is able to recognize and recombine 34 base pair loxp sites in mammalian cells in vitro and in vivo . 
 therefore the cre / loxp system is widely used to conditionally induce mutations in the genome of living cells3,4 . 
 the delivery of active cre recombinase to cells , however , represents a limitation.we describe the psesame vector system , which allows a direct insertion of the gene - of - interest and provides a platform to rapidly clone different domains and tags used within the vector in a convenient and standardized manner . 
 rearranging of the different tags has been shown to modify the biochemical properties of the fusion proteins providing a possibility to achieve higher yield and better solubility . 
 we demonstrate how to express and purify recombinant cell - permeant proteins in and from e. coli . 
 the functionality of the recombinant cre protein is finally validated in cell culture by assessing its intracellular recombinase activity .