(Received for publication, December 4, 1996, and in revised form, February 4, 1997)
From the Institut de Biochimie et Génétique Cellulaires du Centre National de la Recherche Scientifique, UPR 9026, 1 rue Camille Saint-Saëns, F-33077 Bordeaux Cedex, France
The purine-cytosine permease (PCP) is an active transporter located in the plasma membrane of the yeast Saccharomyces cerevisiae. This protein mediates purine (adenine, guanine, and hypoxanthine) and cytosine accumulation in the cell by using an electrochemical potential difference in proton as the energy source.
Various mutant strains, with altered Kt(app) (apparent Michaelis constant of transport) of uptake for one or several bases, have already been selected. Their cloning and sequencing revealed that three of them presented substitutions in the same region of the putative sequence of the PCP: this region might correspond to the hydrophilic segment 371-377 (I-A-N-N-I-P-N). Two mutants displayed single mutations, resulting in only one amino acid residue change (N377I and N374I, respectively), and the other displayed three amino acid substitutions (I371V, I375V, and N377G). Therefore, to analyze the contribution of individual amino acid changes to the phenotype of the complex mutant, single (N377G) and double (I371V,I375V) mutants were constructed by site-directed mutagenesis.
The influence of single mutations in this region was studied by measuring, for adenine, hypoxanthine, and cytosine, the uptake constants on cells and equilibrium binding parameters on plasma membrane-enriched fractions. Uptake and binding constant determinations showed that all the variations observed for the Kt(app) of uptake were correlated with variations of the binding Kd(app) for the corresponding solutes. Thus, our results emphasize the role of the two asparagine residues, located at positions 374 and 377, respectively, in the binding of the bases. In addition, the sole substitution of the 377 asparagine residue by glycine is responsible for the phenotype of the triple mutant.
The effect of pH on the apparent hypoxanthine binding dissociation constant showed that the effects of N377G and N377I changes were, at least partially, due to a shift of the pKa of an ionizable amino acid residue of the unliganded permease. These two amino acid residue changes induced a shift of the pKa of this group in the unliganded, deprotonated permease about two units toward acidic pH. This result suggests that the 371-377 segment might play a key role in the proper three-dimensional structure of the active purine-cytosine permease.
In the yeast Saccharomyces cerevisiae, the purine-cytosine permease (PCP)1 mediates cotransport through the plasma membrane of proton and purine bases (adenine, hypoxanthine, and guanine) or a pyrimidine base (cytosine) (1-6). The FCY2 gene, encoding for PCP, has been cloned (7) and sequenced (8), thus enabling studies of the relationships between the deduced amino acid sequence of the permease (a polytopic protein consisting in 533 amino acid residues with a molecular mass of 58.2 kDa) and the transport mechanism.
An important question for the mechanism of base transport through the
plasma membrane is the characterization of the amino acid residues
involved in the binding of ligands. It has been shown that the 4 bases
bind to the same site with very similar affinities and that their
uptake occurs with equivalent efficencies (3, 9). To identify the
binding site, an interesting approach involves the isolation of mutant
strains with altered affinity constants for the various ligands, as
already described in the case of lactose permease of Escherichia
coli (10-12). Screening of PCP mutants (3), based on competition
on either adenine and cytosine uptake in an Ade strain
(fcy2-21 and fcy2-20 alleles), or cytosine and
5-methylcytosine in an Ura
strain (fcy2-10)
allowed the identification of a region of the protein whose mutations
led to strains with altered Kt(app) (apparent Michaelis constant of transport) of purine bases and cytosine. This
region, according to a hypothetical topologic model (8), is located in
a loop (367-399) connecting two putative transmembrane
helices and
facing the periplasmic space. The cloning and sequencing of these
mutants (13) revealed that one of them (fcy2-21 allele) had
extensive nucleotide replacement, resulting in three amino acid changes
(I371V, I375V, and N377G), whereas two others (fcy2-10 and
fcy2-20 alleles) displayed only one amino acid change (N377I and N374I, respectively). In addition, to analyze the contribution of
each amino acid residue change to the phenotype of the triple mutant
fcy2-21, single (N377G) and double (I371V,I375V) mutants have been constructed by site-directed mutagenesis (13). All these
mutants displayed modifications of the uptake parameters for the
different bases. We have already extensively described the triple
mutant strain and shown that modifications of equilibrium binding
parameters were responsible for the uptake phenotype (9).
The purpose of the present study was to investigate in greater detail the contribution of individual amino acid residue changes localized in the 371-377 hydrophilic segment to the phenotype of the various mutant strains, especially on the base binding step. This was done by measuring, for adenine, cytosine, and hypoxanthine, the uptake activity on intact cells and equilibrium binding parameters on plasma membrane enriched fractions. Moreover, dissociation constants for hypoxanthine and proton binding were calculated by measuring the amount of specifically bound hypoxanthine as a function of the external pH. This approach made it possible to investigate the role of the 371-377 segment in the active conformation of the functional PCP.
Adenine, hypoxanthine, and cytosine were
purchased from Aldrich. Radioactive [8-14C]adenine (1.96 GBq·mmol1) and [8-14C]hypoxanthine (1.88 GBq.mmol
1) were from DuPont NEN and
[2-14C]cytosine (1.96 GBq.mmol
1) from
Moraveck. Restriction endonucleases and enzymes used for constructions
and analyses of plasmids were purchased from New England Biolabs,
Amersham Corp., and Life Technologies, Inc. and used as recommended by
the suppliers. Prestained molecular mass markers were obtained from
Sigma. Horseradish peroxidase-conjugated IgG were from
Pierce. ECL Western blotting kit and hyperfilmTM ECL were obtained from
Amersham. Protein A-Sepharose beads were purchased from Pharmacia
Biotech Inc. LB and yeast nitrogen base without amino acid were from
Difco. Scintillation mixtures were Ready Safe from Beckman. All other
materials were of analytical grade.
E. coli HB101 (supE44,
hsdS20(rBmB
),
recA13, ara-14, proA2, lacY1, galK2,
rpaL20, xyl-5, mtl-1) (14) was used as
carrier for the plasmids described. Characterization and isolation of the yeast strains used in this work have already been described (8,
15). All are derivatives of the NC233-10B (MAT a,
fcy1-1, fcy2
(BglII-KpnI), leu2)
strain. NC233-10B[pJDB] is a permease-null strain. Strain
NC233-10B[pAB4] is a permease-proficient strain, carrying
plasmid-encoded multiple copies of the FCY2 gene.
NC233-10B[pAB25] is a permease-proficient strain, carrying
plasmid-encoded multiple copies of the mutated fcy2-21
allele.
E. coli strains were grown at 37 °C, in LB liquid or
solid medium, with or without ampicillin. Yeast strains were grown at 28 °C under agitation in a yeast nitrogen base without amino acid medium (6.75 g·l1) containing 2% glucose (w/v) and 25 mM sodium phtalate, pH 4.5.
Accurate determination of uptake and binding parameters can be performed only on overexpressed PCP-containing strains. Thus, all the FCY2 alleles (wild type and mutated) were subcloned onto the multicopy pJDB207 plasmid (16). Plasmids pTF1 and pTF2 were constructed by replacing the restriction fragment BglII-SphI (1.92 kilobase pairs) in the FCY2 open reading frame of pAB4 (8) by the corresponding fragment from the pJC5 or pJC6 plasmids (13), respectively. Plasmids pTF2-10 and pTF2-20 were constructed by replacing the restriction fragment MluI-ApaLI (1.76 kilobase pairs) of pAB4 by the corresponding fragment from the plasmids pPZ1-10 or pPZ1-20 (13), respectively. Double-stranded plasmid DNA prepared by WizardTM Midipreps (Promega) were sequenced using the dideoxynucleotide termination method (17) and synthetic sequencing primers after alkaline denaturation (18).
E. coli cells were transformed according to Cohen et al. (19) and yeast cells according to Gietz et al. (20).
To simplify the text, the NC233-10B transformed yeast clones used in this study will be referred to only by the plasmid they harbor.
Yeast Plasma Membrane PreparationPlasma membrane-enriched
fractions (PMF) were isolated as described previously (21). Cells
(2.8-3.2×107 cells·ml1) were disrupted
with glass beads, and the homogenate was submitted to differential
centrifugations. Mitochondrial material was eliminated by acid
precipitation (22).
Initial rates of uptake (adenine, hypoxanthine, and cytosine) were measured at various solute concentrations by a filtration technique, essentially as described previously (9). The maximal rate of uptake (Vmax) and the apparent Michaelis constant of transport (Kt(app)) were calculated by nonlinear regression analysis of the initial rates of uptake versus the solute concentration, using the method of Cleland (23).
Ligand binding measurements were performed by a centrifugation technique (9) in 50 mM sodium citrate, pH 4.5, containing 100 mM NaCl. The maximal amount of specifically bound ligand (Bmax) and the apparent half-saturation constant of solute binding (Kd(app)) were calculated by nonlinear regression analysis of the saturation curve.
Guanine was not used in this study because of its poor solubility under our experimental conditions.
Hypoxanthine Binding as a Function of External pHHypoxanthine binding measurements as a function of the external pH were done as described previously (24), with slight modifications. pH variations from 3.5 to 6.2 were obtained by mixing two buffer solutions (50 mM citric acid containing 100 mM NaCl, pH 1.9, and 50 mM sodium citrate containing 100 mM NaCl, pH 8.1) at a given hypoxanthine concentration (11-79 µM depending on the strain). Under these conditions, the ionic strength of the incubation medium varies between 150 and 350 mM. In control experiments, it was found that, at a given pH, these ionic strength variations in the incubation medium had no effect on the amount of specifically bound hypoxanthine.
In a previous work (24), it was shown that a random mechanism with a
stoichiometry 1/1/1 (proton/base/site) was the best model to fit the
experimental data obtained for the binding of proton and hypoxanthine
to plasma membrane fractions. In this random process (Fig.
1), four constants are defined. Kd and Kd, are respectively the dissociation constants of hypoxanthine from the protonated and deprotonated PCP.
K1 and K2 are,
respectively, the acidic dissociation constants of the proton from a
group of the free and the liganded PCP. Calculations of these constants
were done using the following equations.
![]() |
(Eq. 1) |
![]() |
(Eq. 2) |
![]() |
(Eq. 3) |
Equation 2 can be written in a logarithmic form (as follows).
![]() |
In this work, we have used the plot pKd(app)
versus pH, which is very convenient to visualize directly
the effects of the mutations. This curve admits two asymptotic values
(the plateaus), one at very acidic pH values (pH pK1 and pK2) whose value
is pKd and the other at very basic pH values (pH
pK1 and pK2) whose
value is pKd + pK1-pK2. The midpoint is
observed when pH = (pK1+
pK2)/2. The slope at this midpoint is correlated
with the difference between pK1 and
pK2 and shows a lower limit value of
1,
i.e. when pK2
pK1 (which means at least a 3 pH unit difference
between the two pKa values or
K1/K2 > 1000) and an
upper limit value of 0 (when pK2
pK1). The drawn tangent at the midpoint gives
intercepts with the upper and lower asymptotes that are
pK1 and pK2,
respectively.
PMF samples (1 µg of protein) were separated on Tris/Tricine SDS-polyacrylamide gel electrophoresis on 12.5% polyacrylamide resolving gels (25) and then transferred to polyvinylidene difluoride membranes (Amersham) for Western blot analyses (26). The blots were incubated with either purified anti-N-terminal PCP (27) or anti-H+-ATPase (a generous gift of J. Nader, University of Louvain-la-Neuve) as primary antibodies. Peroxidase-conjugated anti-rabbit IgG were used as secondary antibodies. Reacting materials were visualized using the ECLTM Western blotting protocol according to the manufacturer's guidelines (Amersham).
MiscellaneousCell concentrations were determined by
measuring the turbidity of the culture medium at 550 nm (an absorbance
unit corresponding to 2 × 107
cells·ml1). Protein concentrations were determined by
the method of Lowry (28) using bovine serum albumin as standard.
The NC233-10B strain, carrying a chromosomal deletion at the FCY2 locus, was transformed with the plasmids presented in Table I.
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Expression of the relative amounts of PCP among the various strains was
checked by using Western blot detection of both PCP and plasma membrane
H+-ATPase on PMF samples (Fig. 2). With
anti-N-terminal PCP antibodies, a unique band was detected in the range
of 47-52 kDa for the overexpressed wild type strain (Fig. 2,
lane 7) and all of the overexpressed mutant strains (Fig. 2,
lanes 1-5). This band, which was not visible for the
permease-null strain (Fig. 2, lane 6), corresponds to PCP.
Thus, the different mutations did not seem to affect PCP targeting to
the plasma membrane.
Scanning of the bands corresponding to PCP and to H+-ATPase, used as an internal standard, showed that there was no significant difference in the overexpression level of PCP (40-fold overexpression as compared with the wild type) among the various multicopy strains studied (data not shown).
Purine Bases and Cytosine Uptake Measurements with Intact CellsThe uptake constants are presented in Table II.
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Despite a quite similar expression level of PCP among the various strains of this study (Fig. 1), significant variations were observed for Vmax. For a given strain, these variations could be in relation to the base transported; and for a given base, variations could be in relation to the strain. In addition, these Vmax variations, in contrast to those observed for the Kt(app) values (see below), could be either increases or decreases and, for a given strain, could be more or less pronounced depending on the transported solute. For example, if one compares the pTF2-10 cells and the wild type, there was a Vmax decrease for adenine (2-fold) and a Vmax increase for cytosine (2-fold) for the mutant strain, whereas for hypoxanthine, Vmax remained the same.
Since Vmax is a complex parameter related to the efficiency of different steps of the solute translocation process, it is not yet possible to explain these variations.
fcy2-21 Allele DerivativesThe main effect of the triple substitution (I371V, I375V, and N377G) in pAB25 cells, as compared with the pAB4 strain, was a large increase in Kt(app) for two solutes (6- and 14-fold for cytosine and hypoxanthine, respectively) and no change for adenine (9, 24).
For pTF1, which has two (I371V, I375V) of the mutations of the pAB25 strain, the Kt(app) for the 3 bases were quite similar to those observed for the wild type. The main effect of the two amino acid residue changes was a slight increase (1.5-fold) in the Kt(app) for cytosine. For the pTF2 simple mutant strain (N377G), Kt(app) values for cytosine and hypoxanthine were dramatically increased (6.4- and 25.6-fold, respectively), whereas the Kt(app) for adenine remained the same, as compared with the wild type. Thus, the simple mutant pTF2 behaves like the triple mutant strain pAB25, whereas the double mutant pTF1 resembles the wild type.
fcy2-10 AlleleFor pTF2-10 cells, the substitution N377I resulted in a large increase in the Kt(app) for the 3 bases, as compared with the pAB4 wild type strain (9-, 19-, and 23-fold for cytosine, hypoxanthine, and adenine, respectively). Kt(app) values for cytosine and hypoxanthine of this mutant were of the same order of magnitude as those observed for the pTF2 strain, the mutant for which the same Asn residue was replaced by Gly instead of Ile.
fcy2-20 AlleleIn pTF2-20 cells (N374I), Kt(app) values for the 3 bases were also increased, more dramatically than already observed for the pTF2-10 cells. In fact, this mutation was the amino acid residue change that induced the largest increases in Kt(app) values for cytosine and hypoxanthine (26- and 61-fold, respectively).
It is noteworthy that, for fcy2-10 and fcy2-20 alleles, the Kt(app) of adenine was affected, although this was not the case for the fcy2-21 allele and its derivatives.
Equilibrium Binding Measurements on PMFFor all strains and solutes tested, saturation curves showed only one class of binding site (data not shown). The Kd(app) and Bmax, obtained by nonlinear regression of the saturation curves, are displayed in Table III. With a given strain, the Bmax measured is quite similar for the 3 bases tested. On the contrary, Bmax values obtained for the different strains showed significant variations. The dispersion of this parameter reflects the poor reproducibility we observed on numerous large-scale PMF preparations. Thus, owing to this heterogeneity of PMF preparations, no comparison will be made for this parameter among the various strains.
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In terms of Kd(app) and as already observed with base uptake, the pTF2 mutant (N377G) behaves in a way very similar to that of the triple mutant, while the pTF1 double mutant (I371V,I375V) behaves like the wild type, even though Kd(app) values for cytosine and hypoxanthine are slightly increased.
With pTF2 PMF, N377G replacement resulted in a small but significant increase in Kd(app) for adenine (2.8-fold) and a large increase for cytosine and hypoxanthine (24-fold for these 2 bases) as compared with the wild type.
fcy2-10 and fcy2-20 AllelesFor pTF2-20 and pTF2-10 (N374I and N377I, respectively), Kd(app) were dramatically increased for the 3 bases tested. Kd(app) for adenine was the same in these two strains and was 2 orders of magnitude higher than that of the wild type. This decrease in affinity was so great for pTF2-20 PMF that any specific binding could hardly be measured for cytosine and hypoxanthine.
Equilibrium Binding Measurements of Hypoxanthine on PMF as a Function of External pHEquilibrium binding measurements of hypoxanthine at different pH values were done on PMF from pTF1, pTF2, and pTF2-10, at a constant hypoxanthine concentration (11, 71, and 79 µM, respectively).
As already observed for pAB4 and pAB25 strains (24), the amount of
hypoxanthine specifically bound to PMF from pTF1, pTF2, and pTF2-10
increased continuously when the proton concentration was varied from
0.6 to 350 µM (Fig. 3). Since
Bmax remained the same at any proton
concentration tested (data not shown), these variations in the amount
of specifically bound hypoxanthine reflected Kd(app)
variations. Moreover, these Kd(app) variations
cannot be ascribed to a significant change in the hypoxanthine ionization level in the range of pH tested.
Plots of pKd(app) versus pH are presented
in Fig. 4. When possible, dissociation constants of the
hypoxanthine binding model were estimated by nonlinear regression using
Equation 2 and are displayed in the inset of Fig. 4.
For pAB4, we have shown that Kd was 1.7 µM and that an amino acid residue, displaying a pK1 = 3.8, might be involved in PCP protonation (24); this pKa was shifted to a more alkaline value (pK2 = 4.8) when hypoxanthine was bound. For pAB25, the main effects of the triple mutation were: (i) a large decrease in the affinity of PCP for hypoxanthine (Kd of 14.4 µM); (ii) a shift in the pK1 of the amino acid residue toward a more acidic pH (pK1 < 3.1).
fcy2-21 DerivativesFig. 4 clearly shows that the sole
substitution N377G (pTF2) can account for the phenotype of the triple
mutant pAB25. This modification induced such a shift of the curve
toward a more acidic pH that the top plateau could not be reached under
our experimental conditions. Because of the flocculation of PMF,
binding measurements could not be made below pH = 3.5. Consequently, accurate values of K1 and
Kd for pTF2 were quite difficult to estimate. Anyway, the data show that pK1 was shifted
toward more acidic pH (pK1 < 2.7) than in the
case of the wild type, whereas pK2 remained
unchanged (pK2 = 4.3). Kd did
not appear to be significantly modified by the N377G substitution, and
Kd was dramatically increased.
Owing to the poor binding affinity of PCP for
hypoxanthine with PMF from pTF2-10 strain (displaying the N377I
substitution) at pH 4.5 (Table II) and since Kd(app)
increased with increasing proton concentration, specifically bound
ligand could not be accurately measured with this strain at pH > 4.5. This, together with the impossibility of doing binding
measurements at pH below 3.5, made the calculations of the dissociation
constants for hypoxanthine binding highly speculative. Clearly,
horizontal plateaus were not reached. Anyway, as compared with the wild
type, the curve was clearly shifted toward more acidic pH. In addition, experimental data were scattered in a narrow window rendering impossible any fit with the theoretical curve. Nevertheless, within this window, pKd(app) varied almost linearly as a
function of pH and with a slope of about 0.8 (i.e. closed
to
1). This suggests that the difference between
pK2 and pK1 is at least
equal to 2.5 pH units and that pK1 is lower than
3. Thus, the hypoxanthine binding profile observed with pTF2-10 PMF is
partially due to a modification of the acidic dissociation constants of
the proton (K1 and/or K2)
of PCP. This behavior is very similar to that induced by the N377G
substitution in the pTF2 strain.
Since the affinity of the PCP for hypoxanthine was too low (Table III), these experiments were not feasible with PMF from the pTF2-20 strain.
The study of uptake and binding parameters on several PCP mutants displaying amino acid changes in the 371-377 segment has allowed us to show clearly that the observed phenotype of the double mutant strain pTF1 (I371V,I375V) is the same as the wild type one and that the simple mutant pTF2 (N377G) has the phenotype of the triple mutant pAB25 (I371V,I375V,N377G). Therefore, the phenotype of the triple mutant is only due to the substitution N377G. This amino acid residue change leads to PCP being able to discriminate the 3 bases, and this effect seems, at least partially, to be due to modification of the binding step for cytosine and hypoxanthine. Moreover, as the N377G substitution did not dramatically modify the Kd(app) for adenine, we may assume that the amido group of this Asn residue does not play a crucial role in the binding of adenine.
Another selected mutant showing the N377I substitution (pTF2-10) was characterized by larger increases in both Kt(app) and Kd(app) for hypoxanthine than that observed for the N377G substitution and, in addition, a large increase in these parameters for adenine. Thus, even though the amido group of Asn-377 does not play a crucial role in the binding of adenine, its substitution by a large hydrophobic residue like Ile had a significant effect on the Kd(app) of the purine bases. This suggests that steric hindrance of the Ile residue would affect the access of adenine to its binding site. Finally, whereas the substitutions N377G and N377I had distinct effects on purine bases, they still had the same effect on cytosine binding. A possible explanation for these results could be that owing to its structure, cytosine might not show the same sensitivity to steric hindrance of the residue 377.
Interestingly enough, another spontaneous mutant (pTF2-20, displaying the N374I substitution) also showed an Asn change in the same region. If we compare the results obtained with the pTF2-10 mutant, it appears that N374I and N377I displayed quite similar phenotypes, N374I being the change for which the Kt(app) and Kd(app) for cytosine and hypoxanthine were the most dramatically increased. These results raise the question as to whether these two residues belonging to the sequence segment 371-377 (I-A-N-N-I-P-N) play a similar role for the proper conformation of the active PCP.
Moreover, all the results obtained with these mutants were modifications of uptake and binding constants, which implied changes of the tertiary structure of the active PCP. We have investigated this hypothesis by studying the effects of these two Asn residue replacements on the binding of hypoxanthine as a function of external pH.
Again, our results on pTF1 and pTF2 confirm that the sole N377G substitution can account for the phenotype of the triple mutant pAB25.
Surprisingly, for pTF2, Kd does not seem to be
significantly modified. The main effect of the N377G substitution is a
dramatic increase in K1 and
Kd. Thus, it seems that this amino acid residue
change has a dramatic effect on the free, deprotonated mutated PCP,
which binds both hypoxanthine and proton with lower affinities than the
wild type forms.
The nature of the PCP amino acid residue whose pKa is modified by the binding of the base remains to be elucidated. However, from the calculated pK1 for the wild type strain (pK1 = 3.8), it should be a carboxylic group. Since the pKa value of an amino acid residue reflects its environment, the shift of pK1 due to N377G substitution may be explained by a modification of the environment of an amino acid residue of free and deprotonated PCP. This shift could be due to a conformational change (nearby or at a distance) induced by the replacement of Asn-377 by Gly. Many kinds of electrostatic interactions may be involved in the pKa variations of an amino acid residue in a protein, as compared with its pKa in pure water (29, 30). The shift of pK1 toward more acidic pH may reflect the protection of the assumed carboxylic group from complete protonation, owing to stabilization of its basic form. In this hypothesis, a possible explanation for the observed phenomenon is that the N377G substitution may induce exposure of this ionizable group toward a more hydrophilic environment. However, we still do not know whether this group is located near or far from the Asn-377 residue. In contrast to what is observed for pK1, it should be pointed out that pK2 is not modified by the N377G substitution. This means that hypoxanthine binding restores in the mutated PCP an environment of this ionizable group very similar to that of the wild type.
The effect of N377I change on the proton dissociation constants of the PCP is not well determined. Nevertheless, since pK1 is largely shifted toward acidic pH (pK1 < 3), it seems that this mutation also modifies the environment of the ionizable group. However, in this case, it is not possible to know if this replacement affects only the carboxylic group environment in the free permease and/or also in the liganded permease, since pK2 could not be estimated.
Taken altogether, these results argue for the role of the 371-377 segment in the maintenance of a functional conformation of the PCP. This is strengthened by the presence of a Pro residue at position 376. Site-directed mutagenesis constructs in this region are under study and should allow a more accurate investigation of the role of this segment in the tertiary structure of the active PCP.
Our present aim is to determine whether or not this segment is part of the base binding site of PCP. Specific covalent labeling of PCP by 8-azido-[2-3H]adenine (7, 21) and subsequent isolation of radiolabeled peptides of the permease together with mutant strain analyses would allow better characterization of some of the constitutive elements of the binding site.
We gratefully acknowledge Dr. J. De Montigny and M. L. Straub (Laboratory Microbiologie Génétique, URA 1481, Strasbourg, France) for their generous gift for the pPZ1-10, pPZ1-20, pJC5 and pJC6 plasmids. We also acknowledge R. Cooke for improving the grammar.