We present an analysis of renormalization effects for a class of grand unified theories which contain the $\mathrm{SU}{(2)}_{L}\ifmmode\times\else\texttimes\fi{}\mathrm{SU}{(2)}_{R}\ifmmode\times\else\texttimes\fi{}\mathrm{U}{(1)}_{B\ensuremath{-}L}$ left-right-symmetric low-energy electroweak model. Previously obtained predictions for the proton lifetime are improved at the two-loop level and we find ${10}^{32}\ensuremath{\le}{\ensuremath{\tau}}_{p}\ensuremath{\le}{10}^{39}$ yr for $50\ensuremath{\le}{\ensuremath{\Lambda}}_{\stackrel{-}{\mathrm{MS}}}\ensuremath{\le}400$ MeV ($\stackrel{-}{\mathrm{MS}}$ indicates the modified minimal-subtraction scheme). In addition, we calculate the quark-lepton mass ratios and the enhancement factors for all the leading baryon-number-violating operators. Implications of our results for the selection rules in baryon decay are also presented.