  We show that the moduli space $M$ of holomorphic vector bundles on $CP^3$
that are trivial along a line is isomorphic (as a complex manifold) to a
subvariety in the moduli of rational curves of the twistor space of the moduli
space of framed instantons on $\R^4$, called the space of twistor sections. We
then use this characterization to prove that $M$ is equipped with a
torsion-free affine connection with holonomy in $Sp(2n,\C)$.
