  The response of a semiflexible polyelectrolyte chain to stretching in the
regimes of moderate and weak screening is studied theoretically, with a special
focus on DNA experiments. By using the nonlinear Poisson--Boltzmann description
of electrostatic self--interactions of the chain, we explicitly demonstrate the
applicability of the concept of effective charge to certain aspects of the
problem. This charge can be extracted from the far--field asymptotic behavior
of electrostatic potential of the fully aligned chain. Surprisingly, in terms
of the effective charge, the electrostatically renormalized persistence length
can be formally described by the classical Odijk-Skolnick-Fixman (OSF) formula,
whose domain of applicability is limited to linearized Debye Huckel (DH)
approximation. However, the short--length behavior of the chain in the
nonlinear regime deviates from the of DH--based result, even upon the charge
renormalization. This difference is revealed in calculated stretching curves of
strongly charged DNA, and our results are in good agreement with the recent
experiments. In the limit of weak screening we predict the elastic response to
have a distinctive two-stage character, with a peculiar intermediate
"unstretchable" regime.
