A shifty stop for a hairy tail

Abstract

The tail apparatus of the bacteriophage SPP1 is an extraordinary approximately 1600-A-long molecular machine. The tail mediates attachment of the virus to the host surface receptor, as well-as ejection of the viral genome into the host. The distal tip of the tail binds the extracellular ectodomain of the Bacillus subtilis receptor YueB, while the tail tube provides a conduit to funnel the viral genome into the host. This process, which culminates with the ejection of the approximately 44 kb of viral DNA across the thick, cell envelope of the Gram-positive bacterial cell, takes place in a time scale of seconds to minutes and represents a remarkable example of biotransformation. In this issue of Molecular Microbiology, Auzat et al. provide compelling evidence that the two major structural proteins of the SPP1 tail, gp17.1 (approximately 19.1 kDa) and gp17.1* (approximately 28 kDa), share a common N-terminal sequence, and that gp17.1* is generated by a translational frameshift in the gene 17.1. The extra domain fused to gp17.1* is synthesized by a +1 programmed translational frameshift at the end of gene 17.1, which leads to the synthesis of approximately one gp17.1* for every three equivalents of gp17.1. This finding extends our current knowledge of translational frameshifts and provides a framework to understand how Siphoviridae phages like SPP1 have developed long-tail machines using only two major structural proteins.

Figure 1

Common mechanisms of viral frameshift.
A. In the standard model of a −1 frameshift, the ribosome encounters the sequence X.XXY.YYZ and is paused usually as a result of a 3′ secondary structure in the mRNA (e.g. a stem‐loop or pseudoknot). During this pause, the ribosome slips backwards one base, with both the A‐site and P‐site re‐annealing, changing only the base pairing of the wobble base.
B. In the +1 frameshift, the ribosome pauses immediately before a stop codon, normally waiting for a rare tRNA^Pro (shown as dashed) at the proline ‘CCC’ codon. This leads the ribosome to move forward one base, and in the case of the rare tRNA^Pro, the A‐site finds the more common ‘CCU’ codon rather than the rare ‘CCC’.