  A photometric redshift analysis of galaxies in the field of the
wide-separation gravitational lens MG2016+112 reveals a population of 69
galaxies with photometric redshifts consistent with being in a cluster at the
redshift of the giant elliptical lensing galaxy, z=1.00. The Ks-band luminosity
function of the cluster galaxies is well represented by the Schechter function
with a characteristic magnitude Ks*=18.90 (-0.57/+0.45) and faint-end slope
alpha=-0.60 (-0.33/+0.39), consistent with what is expected for a passively
evolving population of galaxies formed at high redshift, zf>2. From the total
Ks-band flux of the cluster galaxies and a dynamical estimate of the total mass
of the cluster, the restframe Ks-band mass-to-light ratio of the cluster is
derived to be M/L=27(-17/+64)(h_50)(M/L)_{sun}, in agreement with the upper
limit derived earlier from Chandra X-ray observations and the value derived
locally in the Coma cluster. The cluster galaxies span a red sequence with a
considerable scatter in the colour-magnitude diagrams, suggesting that they
contain young stellar populations in addition to the old populations of
main-sequence stars that dominate the Ks-band luminosity function. This is in
agreement with spectroscopic observations which show that 5 out of the 6
galaxies in the field confirmed to be at the redshift of the lensing galaxy
have emission lines. The projected spatial distribution of the cluster galaxies
is filamentary-like rather than centrally concentrated around the lensing
galaxy, and show no apparent luminosity segregation. A handful of the cluster
galaxies show evidence of merging/interaction. The results presented in this
paper suggest that a young cluster of galaxies is assembling around MG2016+112.
