I present results from a suite of cosmological radiation hydrodynamics simulations that focus on the transition from Population III star formation to the first galaxies. Each simulation captures the radiative and chemical feedback from ~300 Population III stars, leading to the formation of a 10^9 solar mass galaxy at redshift 7. We have investigated the variations in galaxy properties when changing the Pop III characteristic mass, critical metallicity, UV backgrounds, metal cooling, and feedback from radiation pressure. One constant result from all the simulations is a metallicity floor between [Z/H] = -3 to -4. We show that momentum transfer from ionizing radiation plays an important role in providing turbulent support and mixing metals, preventing the overproduction of stars and metals. This results in a stellar population with a tight metallicity distribution function centered at [Z/H] = -2, agreeing with the observed luminosity-metallicity relation in dwarf galaxies.