  The Standard Model indicates the realization of grand unified structures in
nature, and can only be viewed as an effective theory below a higher energy
cutoff. While the renormalizable Standard Model forbids proton decay mediating
operators due to accidental global symmetries, many extensions of the Standard
Model introduce such dimension four, five and six operators. Furthermore,
quantum gravity effects are expected to induce proton instability, indicating
that the higher energy cutoff scale must be above 10^{16}GeV. Quasi-realistic
heterotic string models provide the arena to explore how perturbative quantum
gravity affects the particle physics phenomenology. An appealing explanation
for the proton longevity is provided by the existence of an Abelian gauge
symmetry that suppresses the proton decay mediating operators. Additionally,
such a low-scale U(1) symmetry should: allow the suppression of the left-handed
neutrino masses by a seesaw mechanism; allow fermion Yukawa couplings to the
electroweak Higgs doublets; be anomaly free; be family universal. These
requirements render the existence of such U(1) symmetries in quasi-realistic
heterotic string models highly non-trivial. We demonstrate the existence of a
U(1) symmetry that satisfies all of the above requirements in a class of
left-right symmetric heterotic string models in the free fermionic formulation.
The existence of the extra Z' in the energy range accessible to future
experiments is motivated by the requirement of adequate suppression of proton
decay mediation. We further show that while the extra U(1) forbids dimension
four baryon number violating operators it allows dimension four lepton number
violating operators and R-parity violation.
