  Let $X$ be a compact connected Riemann surface of genus $g \geq 2$, and let
${\mathcal M}_{\rm DH}$ be the rank one Deligne-Hitchin moduli space associated
to $X$. It is known that ${\mathcal M}_{\rm DH}$ is the twistor space for the
hyper-K\"ahler structure on the moduli space of rank one holomorphic
connections on $X$. We investigate the group $\operatorname{Aut}({\mathcal
M}_{\rm DH})$ of all holomorphic automorphisms of ${\mathcal M}_{\rm DH}$. The
connected component of $\operatorname{Aut}({\mathcal M}_{\rm DH})$ containing
the identity automorphism is computed. There is a natural element of
$H^2({\mathcal M}_{\rm DH}, {\mathbb Z})$. We also compute the subgroup of
$\operatorname{Aut}({\mathcal M}_{\rm DH})$ that fixes this second cohomology
class. Since ${\mathcal M}_{\rm DH}$ admits an ample rational curve, the notion
of algebraic dimension extends to it by a theorem of Verbitsky. We prove that
${\mathcal M}_{\rm DH}$ is Moishezon.
