  Aims. The high energy spectrum of 3C 273 is usually understood in terms of
inverse-Compton emission in a relativistic leptonic jet. This model predicts
variability patterns and delays that could be tested with simultaneous
observations from the radio to the GeV range. Methods. The instruments IBIS,
SPI, JEM-X on board INTEGRAL, PCA on board RXTE, and LAT on board Fermi have
enough sensitivity to follow the spectral variability of 3C 273 from the keV to
the GeV. We looked for correlations between the different energy bands,
including radio data at 37 GHz collected at the Mets\"ahovi Radio Observatory
and built quasi-simultaneous multiwavelength spectra in the high energy domain
when the source is flaring either in the X-rays or in the {\gamma} rays.
Results. Both temporal and spectral analysis suggest a two-component model to
explain the complete high energy spectrum. X-ray emission is likely dominated
by a Seyfert-like component while the {\gamma}-ray emission is dominated by a
blazar-like component produced by the relativistic jet. The variability of the
blazar-like component is discussed, comparing the spectral parameters in the
two different spectral states. Changes of the electron Lorentz factor are found
to be the most likely source of the observed variability.
