Distant homologues of eukaryotic actin were only recently identified in bacteria. In the rod-shaped Bacillus subtilis, these proteins, members of the MreB family, form helical cables along the cell axis and are required for the maintenance of proper cell shape. Daniel and Errington now show that MreB proteins direct the deposition of cell wall material.
The authors probed for new cell wall material in Bacillus by labeling precursors inserted into peptidoglycan (PG), the major component of the cell wall. New PG was inserted in a helical pattern matching that of an MreB family member, called Mbl. Mutation of Mbl led to loss of the helical pattern in the cell cylinder. It is not clear how Mbl controls PG synthesis, but the authors believe that the MreC and MreD membrane proteins may connect the cytoplasmic cables to the external cell wall machinery.
Additional transient PG synthesis was found at sites where cell division occurred. Cells lacking Mbl survived by acquiring mutations that allowed them to maintain growth from finished division sites. They continued to grow as long as cell division was not inhibited.
Round bacteria such as Streptococcus lacked MreB proteins and inserted new wall material only at sites of division. Two groups of rod-shaped bacteria also lacked MreB-type proteins. The group found that one of these bacteria, Corynebacterium, relied entirely on continuous growth from cell division sites, just like the B. subtilis mbl mutants. MreB-directed wall synthesis allows cells to grow more rapidly, since new material can be added throughout the growing cell. But polar-growing cells may have the advantage of a static wall that can be reinforced by additional modifications to increase its strength. ▪