  Let (P(t)) be the Ornstein-Uhlenbeck semigroup associated with the stochastic
Cauchy problem dU(t) = AU(t)dt + dW_H(t), where A is the generator of a
C_0-semigroup (S(t)) on a Banach space E, H is a Hilbert subspace of E, and
(W_H(t)) is an H-cylindrical Brownian motion. Assuming that (S(t)) restricts to
a C_0-semigroup on H, we obtain L^p-bounds for the gradient D_H P(t). We show
that if (P(t)) is analytic, then the invariance assumption is fulfilled. As an
application we determine the L^p-domain of the generator of (P(t)) explicitly
in the case where (S(t)) restricts to a C_0-semigroup on H which is similar to
an analytic contraction semigroup. The results are applied to the 1D stochastic
heat equation driven by additive space-time white noise.
