<PAGE>
PROSPECTUS SUPPLEMENT DATED JULY 16, 1998
TO PROSPECTUS DATED MAY 8, 1998
$150,000,000
[LOGO]
6% CONVERTIBLE SUBORDINATED NOTES DUE 2005
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THIS PROSPECTUS SUPPLEMENT SHOULD BE READ IN CONJUNCTION WITH THE PROSPECTUS
DATED MAY 8, 1998 (THE "PROSPECTUS"). THE SELLING HOLDERS TABLE ON PAGE 66 OF
THE PROSPECTUS IS HEREBY SUPPLEMENTED TO INCLUDE ADDITIONAL NOTES NOW HELD BY
CERTAIN SELLING HOLDERS AS INDICATED IN THE FOLLOWING TABLE:
<TABLE>
<CAPTION>
NUMBER OF SHARES OF
ADDITIONAL COMMON STOCK
PRINCIPAL AMOUNT ISSUABLE UPON
OF NOTES OWNED AND CONVERSION OF NOTES
NAME OF SELLING HOLDER THAT MAY BE SOLD THAT MAY BE SOLD(1)
- ---------------------------------------------------------------------- ------------------ ---------------------
<S> <C> <C>
BancAmerica Robertson Stephens........................................ $ 3,500,000(2) 123,456
NationsBanc Montgomery Securities..................................... $ 3,205,000(3) 113,051
</TABLE>
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(1) Includes only full shares of Common Stock issuable upon conversion of the
Notes based on an initial conversion price of $28.35 per share (initially
equivalent to a conversion price of 35,273 shares per $1,000 principal
amount of Notes). A cash payment will be made in lieu of any fractional
interest upon conversion.
(2) The holdings of the additional Notes indicated above are as of July 6, 1998.
(3) The holdings of the additional Notes indicated above are as of July 14,
1998. NationsBanc Montgomery Securities has engaged in transactions with and
performed various investment banking and other services for the Company in
the past and may do so from time to time in the future.
