  We investigate magnetohydrodynamic turbulence driven by the magnetorotational
instability (MRI) in Keplerian disks with a nonzero net azimuthal magnetic
field using shearing box simulations. As distinct from most previous studies,
we analyze turbulence dynamics in Fourier (${\bf k}$-) space to understand its
sustenance. The linear growth of MRI with azimuthal field has a transient
character and is anisotropic in Fourier space, leading to anisotropy of
nonlinear processes in Fourier space. As a result, the main nonlinear process
appears to be a new type of angular redistribution of modes in Fourier space --
the \emph{nonlinear transverse cascade} -- rather than usual direct/inverse
cascade. We demonstrate that the turbulence is sustained by interplay of the
linear transient growth of MRI (which is the only energy supply for the
turbulence) and the transverse cascade. These two processes operate at large
length scales, comparable to box size and the corresponding small wavenumber
area, called \emph{vital area} in Fourier space is crucial for the sustenance,
while outside the vital area direct cascade dominates. The interplay of the
linear and nonlinear processes in Fourier space is generally too intertwined
for a vivid schematization. Nevertheless, we reveal the \emph{basic subcycle}
of the sustenance that clearly shows synergy of these processes in the
self-organization of the magnetized flow system. This synergy is quite robust
and persists for the considered different aspect ratios of the simulation
boxes. The spectral characteristics of the dynamical processes in these boxes
are qualitatively similar, indicating the universality of the sustenance
mechanism of the MRI-turbulence.
