a) Not all alpha satellite monomers contain the 17-bp binding motif of
the centromere protein CENP-B. Some HORs, like D7Z1, have a regular, alternating
pattern of CENP-B boxes, so that reiteration of the HOR unit yields a large
array with dense numbers of CENP-B boxes (monomers with white circles).
b) Pentameric SF3 arrays, like that of D11Z1 (cenll.l), DXZ1 (cenX.l),
and the smaller array of D17Z1-B (cenl7.2), have the same number of CENP-B
boxes, or more, as the dimeric arrays, but the CENP-B boxes are irregularly
spaced. These arrays with irregularly spaced CENP-B boxes are equally competent
for centromere assembly.
c) Some HOR arrays, like that of D7Z2 (cen7.2, SF5) have few or no
CENP-B boxes and were thought to be “dead arrays” that lack
centromere potential. However, D7Z2 (cen7.2) that only has one CENP-B box in
monomer 16 can recruit centromere proteins (Thakur and Henikoff, 2018) and assemble a functional centromere
(Hayden et al., 2013, McNulty et al., 2017).