Simplify the following text for a Grade 10 native English-speaking person.
Original: Prior to being acquired by Henderson Private Equity in December 2006 , John Laing plc was listed on the London Stock Exchange and was a constituent of the FTSE 250 Index .
Simplified: Before being bought by Henderson Private Equity in December 2006 , John Laing plc was listed on the London Stock Exchange and was part of the FTSE 250 Index .