  To investigate further the comparison between AGN and black hole X-ray
binaries, we have studied the main X-ray variability properties of the Seyfert
1 Galaxy Markarian 335. We put particular emphasis on the X-ray time lags,
which is a potentially important diagnostic of physical models. From a 100 ksec
observation by XMM-Newton we show that the power spectrum of this source is
well fitted by a bending power law model, and the bend time-scale Tb is
precisely at the value predicted by the Tb vs Hbeta line-width relation of
McHardy et al. Variations in different energy bands show time-scale dependent
time lags, where higher energy bands lag lower ones. The lag, tau, varies as a
function of the Fourier frequency, f, of the variability component in the light
curves as tau propto 1/f at low frequencies, but there is a sharp cut-off in
the lags at a frequency close to the bend frequency in the power spectrum.
Similar behaviour is seen in black hole X-ray binary systems. The length of the
time lags increases continuously with energy separation, in an almost
log-linear relation. We show that the lag spectra can be produced by
fluctuations propagating through the accretion flow as long as the energy
spectrum of the X-ray emitting region hardens towards the centre.
