sentence: Twenty-four hours after transfection , monolayers of cells were cultured an additional 3 or 24 h in the presence or absence of tetracycline and imaged on a Nikon ECLIPSE TE 2000-S equipped with a charge-coupled device camera , a controlled DeltaRAM monochromator on the excitation side and a beam splitter ( Optical Insight , St. Cloud , MN ) on the emission side fitted with a 505DCRX dichroic and two emission filters , D480 / 30 and D535 / 40 .