The thermal evolution of low metallicity gas during the protostellar collapse is described. By using this result, the fragmentation mass scale is evaluated. Particular attention is paid to the critical metallicity where transition from very massive to low-mass star formation occurs. For this transition, metals must be in a form of dust. Its cooling enables low-mass fragmentataion for metallicity higher than [M/H]=-6 - 51. However, there is a pocket in the metallicity range -5≲[M/H]≲-4 where the low-mass fragmentation is prohibited by sudden heat injection due to three-body H2 formation. This may explain observed scarcity of ultra metal-poor stars in this metallicity range, as well as a clear cutoff at metallicity of [M/H]≃-4, provided that the dust is already present in such low metallicity environments.