Chlorophyll apoprotein accumulation in higher plant chloroplasts is controlled by light-dependent chlorophyll' formation. Dark-grown plants lack chlorophyll and chlorophyl apoproteins. However, the plastid genes' encoding the chlorophyll apoproteins are transcribed; chlorophyll apoprotein mRNA accumulates and associates with polysomes in plastids of dark-grown plants. Pulse-labeling assays revealed a population of short-lived proteins in plastids of dark-grown plants. One of these transiently labeled proteins was CP43, a chlorophyll apoprotein associated with photosystem II. Plse chase assays showed that newly synthesized CP43 was rapidly degraded in plastids of dark-grown plants, which lack chlorophyll. In contrast, CP43 synthesized in plastids from illuminated plants was stable. The synthesis of D1, a chlorophyll apoprotein of the photosystem II reaction center, was also analyzed in plastids of dark-grown and illuminated plants. Radiolabel accumulation into full-length D1 was only 'detected in plastids of illuminated plants. However, Dl translation intermediates of 15-25 kDa were detected in both plastid populations. Pulse-chase assays showed that the 15-to 25-kDa D1 translation products were precursors of mature D1 in plastids of illuminated plants. In contrast, in plastids of darkgrown plants, the 15-to 25-kDa translation intermediates were converted into a 23-kDa polypeptide previously suggested to be a proteolytic product of Dl. These results indicate that chlorophyll produced in illuminated plants stabilizes D1 nascent polypeptides, which allows accumulation of mature DM.Chlorophyll is the key chromophore involved in plant photosynthetic light reactions. This chromophore mediates lightdependent charge separation in photosystem I (PSI) and photosystem II (PSII) and serves as the primary lightharvesting pigment in these photosystems. Chlorophyll is noncovalently associated with at least 12 different membrane-bound proteins of the thylakoid. PSII reaction centers contain two plastid-encoded chlorophyll-binding proteins, D1 although radiolabeling studies failed to detect amino acid incorporation into D1, CP43, CP47, and the P700 chlorophyll apoproteins (10). Within 5 min after dark-grown plants are illuminated, amino acid incorporation into D1, CP43, CP47, and the P700 chlorophyll apoproteins can be detected, and the chlorophyll apoproteins begin to accumulate (11,14,20). The light-dependent activation 'of chlorophyll apoprotein accumulation is controlled by protochlorophyllide reductase and requires formation of chlorophyll a (11,20 4038The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

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Detection of pathogens in contaminated food products by PCR can result in false-positive data due to the amplification of DNA from nonviable cells. A new method based on reverse transcription-PCR (RT-PCR) amplification of mRNA for the specific detection of viable Listeria monocytogenes was developed. The expression of three L. monocytogenes genes, iap, hly, and prfA, was examined to determine a suitable target for amplification of RT-PCR. Total RNA from L. monocytogenes was isolated, and following DNase treatment, the RNA was amplified by both RT-PCR and PCR with primers specific for the three genes. Amplicon detection was accomplished by Southern hybridization to digoxigenin-labeled gene probes. The levels of expression of these three genes differed markedly, and the results indicated that the iap gene would provide a good target for development of a specific method for detection of viable L. monocytogenes based on RT-PCR amplification. After a 1-h enrichment, the 371-bp iap-specific product was detected with a sensitivity of ca. 10 to 15 CFU/ml from pure culture. Detection of the 713-bp hly-specific amplicon was ca. 4,000 times less sensitive after 1 h, whereas detection of the 508-bp prfA product showed the lowest level of sensitivity, with detection not observed until after a 5-h enrichment period. The amplification of the iap mRNA was specific for L. monocytogenes. Overall, the assay could be completed in ca. 54 h. The use of RT-PCR amplification for the detection of viable L. monocytogenes was validated in artificially contaminated cooked ground beef. Following a 2-h enrichment incubation, the iap-specific amplification product could be detected in a cooked meat sample that was originally inoculated with ca. 3 CFU/g. These results support the usefulness of RT-PCR amplification of mRNA as a sensitive method for the specific detection of viable L. monocytogenes and indicate that this method may prove useful in the detection of this pathogen in ready-to-eat, refrigerated meat products.

The two-hybrid system was used to test for pairwise interactions between the tobacco vein mottling virus (TVMV)-encoded RNA-dependent RNA polymerase (or NIb protein) and two other TVMV-encoded proteins: the NIa protein, which consists of genome-linked protein (VPg) and proteinase domains, and the viral coat protein (CP). Using this approach, we find that the NIb protein interacts with both the NIa protein and the CP in yeast cells. Moreover, we find that a mutation in the conserved GDD domain of the NIb protein diminishes the NIb-CP interaction but not the NIb-NIa interaction. Likewise, mutations in the vicinity of the NIa protein to which the genomic RNA is covalently attached eliminate the NIb-NIa interaction. We conclude that the NIb protein interacts with the VPg domain of the NIa protein and that this interaction requires a functional RNA attachment site. This interaction may be important for the initiation of viral RNA synthesis in infected cells. We also conclude that the CP interacts with the NIb in a manner that is sensitive in changes in the highly conserved GDD motif. The role of this interaction in the functioning of the NIb protein or the CP is unclear, but may involve regulation of viral RNA synthesis in infected cells.

Mutants of tobacco vein mottling virus (TVMV) were constructed in which the tyrosine residue (Tyr1860) that links the VPg to the viral RNA was changed to phenylalanine or serine or was inverted in position with the adjacent glycine residue. In another mutant, the tyrosine residue nearest to Tyr1860 (Tyr1867) was changed to a phenylalanine residue. The resulting mutants were tested for their ability to infect Nicotiana tabacum plants or protoplasts. The Tyr1860 mutants did not accumulate to detectable levels in infected plants when tested by ELISA and Northern blot analysis. Moreover, the Tyr1860-associated mutants were not infectious in protoplasts, indicating that mutations involving the linking amino acid of the TVMV VPg abolished viral replićation. In contrast to the Tyr1860 mutants, transcripts from the mutation of Tyr1867 to a phenylalanine residue infected both protoplasts and plants. Analysis of progeny RNA from plants inoculated with the Tyr1867 mutant indicated that a reversion to wild type had occurred in systemically infected leaves.