  We present ASAS data starting 25 days before the discovery of the recent type
IIn SN 2010jl, and we compare its light curve to other luminous IIn SNe,
showing that it is a luminous (M_I ~ -20.5) event. Its host galaxy, UGC 5189,
has a low gas-phase oxygen abundance (12 + log(O/H) = 8.2), which reinforces
the emerging trend that over-luminous core-collapse supernovae are found in the
low-metallicity tail of the galaxy distribution, similar to the known trend for
the hosts of long GRBs. We compile oxygen abundances from the literature and
from our own observations of UGC 5189, and we present an unpublished spectrum
of the luminous type Ic SN 2010gx that we use to estimate its host metallicity.
We discuss these in the context of host metallicity trends for different
classes of core-collapse objects. The earliest generations of stars are known
to be enhanced in [O/Fe] relative to the Solar mixture; it is therefore likely
that the stellar progenitors of these overluminous supernovae are even more
iron-poor than they are oxygen-poor. A number of mechanisms and massive star
progenitor systems have been proposed to explain the most luminous
core-collapse supernovae; any successful theory will need to include the
emerging trend that points towards low-metallicity for the massive progenitor
stars. This trend for very luminous supernovae to strongly prefer
low-metallicity galaxies should be taken into account when considering various
aspects of the evolution of the metal-poor early universe. (abridged)
