The hook and basal body are quite diff erent from the filament
(figure 3.42). Slightly wider than the filament, the hook is made of
different protein subunits. Th e basal body is the most complex
part of a flagellum. In the earliest-made transmission electron
micrographs of the basal bodies of E.coli and most other gramnegative
bacteria, the basal body appeared to have four rings—L ring, P ring, S ring, and M ring—connected to a central rod
It is now known that the S ring and M ring are different
portions of the same protein, and they are now referred to
as the MS ring. A later discovery was the C ring, which is on the
cytoplasmic side of the MS ring. Gram-positive bacteria have
only two rings—an inner ring connected to the plasma membrane
and an outer one probably attached to the peptidoglycan