the following examples serve to clarify the invention . all of the molecular biological methods employed , such as polymerase chain reaction , the isolation and purification of dna , the modification of dna with restriction enzymes , klenow fragment and ligase , transformation , etc ., were carried out in the manner known to the skilled person , in the manner described in the literature or in the manner recommended by the respective manufacturers . the polymerase chain reaction was used to amplify the sera gene from escherichia coli strain w3110 ( american type culture collection , atcc27325 ). the oligonucleotides sera - fw : ( seq id no : 3 ) 5 ′- gaa ttc cat atg gca aag gta tcg ctg gag - 3 ′ ndei and sera - rev : ( seq id no : 4 ) 5 ′- aga aag ctt tta tta gta cag cag acg ggc - 3 ′ hindiii the resulting dna fragment was digested with the restriction enzymes ndei and hindiii and the 5 ′ overhangs were filled using klenow enzyme . the dna fragment was subsequently purified by means of agarose gel electrophoresis and isolated using the geneclean ® method ( geneclean ® kit bio101 p . o . box 2284 la jolla , calif ., 92038 - 2284 ). the sera fragment which was obtained in this way was cloned into the expression vector pqe - 70 ( qiagen , hilden , germany ). to do this , the vector was first of all cut with sphi and bamhi and the 3 ′ overhang was digested off using klenow enzyme while the 5 ′ overhang was filled using the klenow enzyme . the vector fragment was then purified and ligated to the sera fragment . the resulting vector is designated pfl209 . after the construct had been verified by sequencing , the escherichia coli strain jm109 ( stratagene , amsterdam , netherlands ) was transformed and corresponding transformants were selected with ampicillin . the bacterial strain escherichia coli jm109 / pfl209 was deposited in the dsmz ( deutsche sammlung fur mikroorganismen und zellkulturen [ german collection of microorganisms and cell cultures ] gmbh , d - 38142 braunschweig ) under number dsm 15628 in accordance with budapest treaty . an inverse polymerase chain reaction was used to carry out the site - specific mutagenesis at codons 349 and 372 of the sera gene . the vector pfl209 described in example 1 was used as the template . the primers the resulting pcr product was circularized by ligation and transformed into the e . coli strain jm109 . finally , sequencing was used to determine the mutation at codon 349 and to check the correctness of the remaining sequence . in principle , the same procedure was used for mutagenizing codon 372 except that the primers in order to determine pgd enzyme activities , and the influence of l - serine on the activities , 100 ml volumes of lb medium ( 10 g of tryptone / l , 5 g of yeast extract / l , 10 g of nacl / l ), which additionally contained 100 mg of ampicillin / l , were in each case inoculated with 2 ml overnight cultures of the strains harboring the plasmid - encoded sera alleles and incubated in a shaker at 30 ° c . and 150 rpm . at an optical density of 1 . 0 , sera expression was in each case induced by adding 0 . 4 mm isopropyl - α - thiogalactoside and the culture was incubated for a further 3 hours . the cells were subsequently harvested by centrifugation , washed and resuspended in 2 ml of buffer ( 100 mm k - phosphate , ph 7 . 0 ; 10 mm mgcl2 ; 1 mm dithiothreitol ). the cells were disrupted using a french press ( spectronic instruments , inc . rochester , n . y ., usa ) at a pressure of 18 000 psi . the crude extracts were clarified by centrifugation at 30 000 g and pgd activity was determined using the mckitrick and pizer test ( 1980 , j . bacteriol . 141 : 235 - 245 ). the following tables show the pgd activities of different mutants and the corresponding inhibitor constants ki . combining the mutations in condons 349 and 372 should show whether the replacements have a synergistic effect on the feedback resistance . a unique hindiii restriction cleavage site between the two mutation sites was used for this purpose . thus , hindiii / hindiii restriction of the vector containing the sera20 allele was used to isolate a 183 bp fragment which corresponds to the 3 ′ end of the sera gene and contains the mutation at codon 372 . this fragment was cloned into a vector which contained the sera40 allele and which had been digested with hindiii / bamhi , with this thereby resulting in a clone which constituted a double mutant . the following table shows the appurtenant enzyme data . accordingly , while a few embodiments of the present invention have been shown and described , it is to be understood that many changes and modifications may be made thereunto without departing from the spirit and scope of the invention as defined in the appended claims .