The conformation and slow internal motions of a DNA plasmid molecule pUC18 (2687 base pairs) are studied by means of depolarized photon correlation spectroscopy. The autocorrelation functions are measured at different scattering angles between 6.7 degrees and 90 degrees and analyzed as heterodyne autocorrelation functions. One obtains both a relaxation time of similar or equal to 140 mu s, which is interpreted as the tumbling rotational diffusion coefficient of the plasmid, and some faster component with relaxation time around 15 mu s. The value of the rotational diffusion relaxation time is in good agreement with the results of Monte Carlo simulations, while the faster decay has a nonsingle exponential decay behavior. The results of measurements made in the depolarized scattering configuration are compared to those obtained from polarized configuration and critically discussed.