Abstract
Ornamental tobacco (Nicotiana langsdorffii X N. sanderae) secretes
a limited array of proteins (nectarins) into its floral nectar.
Careful sodium dodecyl sulfate-polyacrylamide gel electrophoresis
analysis of tobacco nectar revealed that a broad protein band from
61 to 65 kD actually consists of five discrete protein bands. N-terminal
sequencing and tryptic peptide mass spectrometry fingerprint analysis
demonstrated that the upper three bands are isoforms of the same
protein, NEC5 (Nectarin V), whereas the lower two bands, NEC4 (Nectarin
IV), are related to each other but not to NEC5. Reverse transcription-polymerase
chain reaction (RT-PCR) based upon N-terminal sequence of NEC5 generated
a short cDNA that encoded the N terminus of the NEC5 protein. Two
rounds of inverse-PCR using genomic DNA permitted the isolation
of approximately one-half of the coding region of the nec5 gene
along with 787 nucleotides of the 5'-flanking region. This DNA fragment
was used as a probe to isolate a near full-length nec5 clone from
a nectary-derived cDNA library. BLAST analysis identified the nec5
cDNA as a berberine bridge enzyme-like protein. Approximately 40%
of the cDNA sequence corresponded to peptides that were identified
by tryptic peptide mass spectrometry fingerprint analysis of the
NEC5 protein, thereby confirming that this cDNA encoded the NEC5
protein. In-gel assays also demonstrated that NEC5 contains a covalently
linked flavin, and it possesses glucose oxidase activity. RT-PCR-based
expression analyses showed that nec5 expression is limited exclusively
to the nectary gland during late stages of floral development.
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