No mate I,m afraid this isn't a master key and your kinda screwed without one. The master key has 3 buttons.
You can only programme another key to your car from an existing master key so no you can't get another master or even another spare key. The car would have originally have had 2 master keys and 3 spare 'valet' keys.
Sorry for the bad news but the only way to get new keys for you would be to change the ECU, door locks and ignition barrel and this is at a cost of £2000 from Lexus. And this is what you would have to do if you lost your only key. :(