A −1 Ribosomal Frameshift in the Transcript That Encodes the Major Head Protein of Bacteriophage A2 Mediates Biosynthesis of a Second Essential Component of the Capsid

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Abstract
The two major capsid proteins ofLactobacillusbacteriophage A2 share their amino termini. The smaller of these (gp5A) results from translation oforf5and proteolytic processing after residue 123. The larger form (gp5B) originates through a −1 ribosomal frameshift at the penultimate codon oforf5mRNA, resulting in a product that is 85 amino acids longer than gp5A. Frameshifting needs twocis-acting elements: a slippery region with the sequence C CCA AAA (0 frame), and a stem-loop that begins 9 nucleotides after the end of the slippery sequence. Mutations introduced in the slippery sequence suppress the frameshift. Similarly, deletion of the second half of the stem-loop results in drastic reduction of frameshifting. Both gp5A and gp5B appear to be essential for phage viability, since lysogens harboring prophages that produce only one or the other protein become lysed upon induction with mitomycin C, though no viable phage progeny are observed.

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