Fig. 5: Tomographic reconstruction of a M. smithii cell containing MSTV1 virions.

a Cryo-ET raw slices of a M. smithii PS cell containing MSTV1 virions. Red triangles show full capsids, blue triangles represent empty capsids and pink triangles depict arrays of virion tails. Scale bar, 200 nm. Out of 53 visualized M. smithii cells, only one showed evidence of MSTV1 virion assembly. b A segmented and surface-rendering display of the reconstructed tomogram shown in panel 5a. The reconstruction displays the following cellular and viral components: membrane (light green), cell wall (dark green), ribosomes (yellow), empty capsids (blue), full capsids (red), and viral tails (purple). Scale bar, 200 nm. c Schematic representation of the head-tailed virus assembly, as known from bacteriophages. The process initiates with the assembly of an empty procapsid through the binding of multiple copies of the capsid/scaffold complex to the portal protein. Subsequently, the scaffolding protein undergoes proteolytic cleavage by the viral maturation/prohead protease, leading to the expansion of the procapsid. Concomitantly with or prior to the assembly of the expanded procapsid, DNA is translocated into the capsid with the assistance of the packaging complex, resulting in the formation of a mature capsid. The tail is independently assembled and then attached to the mature capsid, thereby producing a mature virion. Schematic modified from ref. ⁴¹ using Corel Draw Graphics Suite 2021. d Closer view of the various stages of capsid assembly visualized in the tomogram shown in panel 5b; I, empty procapsid; II, expanded procapsid, III, mature capsid, and IV, empty extracellular virion. e Surface area of the capsids at the four stages identified in 5d. n corresponds to the number of capsids counted at the different stages. The box plot displays the median (middle line), the 25th and 75th percentiles (box), minimum and maximum values (whiskers), and individual data points (dots). Source data are provided as a Source Data file.