Cloning of cDNA for dihydrodipicolinate synthase --David A. Frisch, Andrew M. Tommey, Burle G. Gengenbach and David A. Somers Dihydrodipicolinate synthase (DHPS) is the branch point enzyme leading to lysine biosynthesis in plants and is a key site for feedback regulation by lysine. Purified maize DHPS is inhibited 50% by 23 uM lysine (MNL 63:106, 1989) and in conjunction with aspartate kinase, imposes a strict limit on the cellular concentration of lysine. As one of the initial steps in establishing a basis for genetic manipulation of lysine biosynthesis in maize, we have isolated a maize cDNA clone for the DHPS enzyme.

We chose to try direct genetic complementation of the Escherichia coli dapA- auxotroph lacking DHPS enzyme activity. The procedure was similar to that used for direct genetic selection of maize glutamine synthetase cDNA (Snustad et al. Genetics 120:1111, 1988). E. coli dapA- cells require diaminopimelate, a lysine precursor, for growth. These auxotrophic cells were transformed by electroporation with size fractionated Black Mexican Sweet cDNA in pUC13. Selection for transformed, complemented cells on minimal medium containing ampicillin resulted in identification of five colonies. Plasmid DNA isolated from each colony yielded complementation frequencies comparable to the transformation frequency. DHPS activity in the five complemented cell lines (Table 1) was inhibited 86-90% by 100 uM lysine as expected of maize DHPS, but DHPS activity in nontransformed wildtype dapA+ cells was inhibited less than 10%. No DHPS activity was detected in dapA- cells.

Table 1. DHPS activity in crude lysates of transformed Escherichia coli strains.
 
 
Specific activity
Inhibition (%)
Cell line
Control
100 uM lysine
 
Wildtype dapA+
130
120
6
Auxotroph dapA-
ND
ND
-
Transformant 1
230
24
89
Transformant 2
160
15
90
Transformant 3
190
22
88
Transformant 4
300
42
86
Transformant 5
260
27
89
ND = not detected. Specific activity values (0.001 OD/min/mg protein) have been multiplied by 106.

The cDNA insert was isolated as an EcoRI fragment from one of the cell lines and both strands were sequenced. The start of the coding region for maize DHPS was established by comparing the amino-terminus amino acid sequence of gel-purified DHPS monomer with the amino acid sequence derived from the nucleotide sequence. The apoprotein for DHPS is expected to have an amino-terminus transit sequence for transport into the plastid, but the purified mature protein is not expected to have the transit sequence. The amino-terminus sequence of Ala-Ile-Thr-Leu-Asp-Asp-Tyr-Leu of the purified protein was identified in the cDNA sequence as indicated in Figure 1. The nearest in-frame 5' ATG was located 162 bp upstream
(designated 1 in Fig. 1) indicating that the transit peptide consisted of 54 amino acids. The complete DHPS coding sequence for the apoprotein was 1140 bp. The predicted molecular weight of the mature protein was 35,854, which was close to the 38,000 Mr estimate obtained from SDS-PAGE.

Figure 1. Representation of maize DHPS cDNA indicating the amino acid sequence of the mature protein and the corresponding cDNA sequence.

The cDNA clone hybridized to a single transcript of about 1400 nucleotides in northern blots of immature embryo and endosperm total RNA. A genomic clone containing at least part of the coding region has been identified in an EMBL-3 library of W22.


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