An ssRNA Phage Single-gene Lysis Protein Targeting MraY Reveals Convergent Evolution Across the RNA-DNA Phage Boundary

Abstract

Small lytic phages of the ssRNA Leviviricetes and ssDNA Microviricetes achieve host lysis by expression of a single protein, designated as an Sgl protein. Four ssRNA Sgls have been shown to inhibit steps in peptidoglycan biosynthesis: three target MurJ, the Lipid II flippase, and one targets MurA, the committed step of the pathway. A fifth Sgl, the E protein of the canonical ssDNA microvirus φX174, inhibits MraY, which catalyzes the synthesis of the first lipid-linked intermediate. This inhibition requires SlyD, a cytoplasmic FKBP-type peptidyl-prolyl cis-trans isomerase. The Sgl of the Caulobacter ssRNA phage φCb5 (Cebevirus halophobicum) has not been characterized. Here, we present physiological evidence that SglφCb5 targets MraY. Moreover, we show that, despite the absence of detectable sequence similarity, SglφCb5 shares several features with the φX174 E protein, including the distribution of hydrophobic and charged residues, a proposed N-out, C-in membrane topology, and retention of lytic activity in an SglφCb5 -LacZ fusion. Furthermore, expression of the Caulobacter cytoplasmic FKBP-type peptidyl-prolyl cis-trans isomerase CC2799 enhances SglφCb5 -mediated lysis in E. coli. These parallels between Sgl function in a DNA phage and an RNA phage either represent remarkably consonant convergent evolution or some evolutionary cross-over event for which there is currently no precedent.

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