Hi,
You have to use a 555, why? Homework?
Ok, one way is to set the 555 up as a monostable, setting the pulse period for slightly less than 1 second but more than 1 second minus one line cycle (maybe half way between would be optimum), triggering it with a gated line frequency signal. Then, use the output of the monostable to lock out the input line frequency using the gate.
This means that the line frequency triggers the 555, but only when the pulse output is at a low logic level. This keeps the 555 sync'ed to the line frequency without using counters. This however does assume that the 555 can be set up accurately enough to be accurate to 1 second plus or minus one half line cycle time (1/100 second for a 50Hz line). It also has to be able to maintain that accuracy over time. Since that's about 1 percent, the 555 might just do it, but i would rather use counters myself.