The penton capsomer of mammalian adenoviruses consists of a trimeric,
long and thin fibre inserted into a pentameric base. The avian adenovi
ruses possess a penton which presents another symmetry mismatch: each
pentameric base is associated with two fibres. Here we have studied th
e morphology of the penton of CELO virus, an avian adenovirus, and we
have determined the sequence of both fibres, one long and one short. T
he short fibre is probably associated with the base in the same way as
the mammalian viral fibres and we will discuss how the long fibre cou
ld be attached. The shafts of all known adenovirus fibres consist of a
series of 15-residue repeats. The avian virus fibres show a more comp
licated and less regular shaft repeat structure with single, double an
d triple repeats. The sequences of the receptor binding (head) domains
of both fibres are very different from all other known fibre head dom
ains and very different from each other, suggesting that the two fibre
s might bind to different receptors. The genome organization of the se
quenced region is rather different from that in human adenoviruses. In
particular, a region homologous to the human virus E3 region was not
found at the position where it normally occurs in the human virus geno
me. (C) 1995 Academic Press Limited