It is noted that every elemnt of $S_n$ is as product of distict cycles and so $S_n$ can be generated by $(i,i+1),~~i=1,2,...,n-1$. In fact for any $(i,j)\in S_n$: $$(i,j)=(i,i+1)(i+1,i+2)...(j-1,j)(j-2,j-1)...(i+1,i+2)(i,i+1)$$ In other words, $$S_n=\langle(i,i+1)\mid~i=1,2,...,n-1\rangle$$ Moreover:
$(i,i+1)^2=1,~i=1,2,...,n-1~~~~$ and $~~~~ \Big((i,i+1)(i+1,i+2)\Big)^3=1,~i=1,2,...,n-2$
and $~~(i,i+1)(j,j+1)=(j,j+1)(i,i+1),~~i,j=1,2,...,n-1,~|i-j|\ge2$
so if we set $$P_n=\langle x_1,x_2,...x_{n-1}\mid R,S,T\rangle$$ wherein $R=\{x_i^2\mid i=1,...,n-1\}$, $S=\{(x_i,x_{i+1})^3\mid i=1,...,n-1\}$ and $T=\{[x_i,x_j]\mid 1\leq i\leq j-1<n-1\}$ then by using the following bijection $S_n$ gets a presentation as above: $$\theta: P_n\to S_n\\ x_i\to(i,i+1)$$