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Molecular and Cellular Biology, October 1999, p. 6980-6990, Vol. 19, No. 10
0270-7306/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.

Mutations Altering the Predicted Secondary Structure of a Chloroplast 5' Untranslated Region Affect Its Physical and Biochemical Properties as Well as Its Ability To Promote Translation of Reporter mRNAs Both in the Chlamydomonas reinhardtii Chloroplast and in Escherichia coli

David C. Fargo, John E. Boynton,* and Nicholas W. Gillham

Developmental, Cell and Molecular Biology Group, Departments of Botany and Zoology, Duke University, Durham, North Carolina 27708

Received 19 May 1999/Accepted 29 June 1999

Random mutations were generated in the sequence for the 5' untranslated region (5'UTR) of the Chlamydomonas reinhardtii chloroplast rps7 mRNA by PCR, the coding sequence for the mutant leaders fused upstream of the lacZ' reporter in pUC18, and transformed into Escherichia coli, and white colonies were selected. Twelve single base pair changes were found at different positions in the rps7 5'UTR in 207 white colonies examined. Seven of the 12 mutant leaders allowed accumulation of abundant lacZ' message. These mutant rps7 leaders were ligated into an aadA expression cassette and transformed into the chloroplast of C. reinhardtii and into E. coli. In vivo spectinomycin-resistant growth rates and in vitro aminoglycoside adenyltransferase enzyme activity varied considerably between different mutants but were remarkably similar for a given mutant expressed in the Chlamydomonas chloroplast and in E. coli. The variable effect of the mutants on aadA reporter expression and their complete abolition of lacZ' reporter expression in E. coli suggests differences in the interaction between the 5'UTR of rps7 and aadA or lacZ' coding regions. Several rps7 5'UTR mutations affected the predicted folding pattern of the 5'UTR by weakening the stability of stem structures. Site-directed secondary mutations generated to restore these structures in the second stem suppressed the loss of reporter activity caused by the original mutations. Additional site-directed mutations that were predicted to further strengthen (A-Uright-arrowG-C) or weaken (G-Cright-arrowA-U) the second stem of the rps7 leader both resulted in reduced reporter expression. This genetic evidence combined with differences between mutant and wild-type UV melting profiles and RNase T1 protection gel shifts further indicate that the predicted wild-type folding pattern in the 5'UTR is likely to play an essential role in translation initiation.

* Corresponding author. Mailing address: Developmental Cell and Molecular Biology Group, Departments of Botany and Zoology, Box 91000, Duke University LSRC Bldg., Research Dr., Durham, NC 27708. Phone: (919) 613-8157. Fax: (919) 613-8177. E-mail: jboynton{at}acpub.duke.edu.

Molecular and Cellular Biology, October 1999, p. 6980-6990, Vol. 19, No. 10
0270-7306/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.


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