<PAGE>
PROSPECTUS SUPPLEMENT DATED AUGUST 14, 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 SELLING HOLDERS AS
INDICATED IN THE FOLLOWING TABLE:
<TABLE>
<CAPTION>
NUMBER OF SHARES OF
AGGREGATE 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>
Shepherd Investment International, Ltd................................ $ 10,675,000(2) 376,543
</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 Notes indicated above are as of July 31, 1998.
The preceding table has been prepared based on information furnished to the
Company by or on behalf of the Selling Holders. With respect to each Selling
Holder identified in the Prospectus and each Prospectus Supplement, the
principal amount set forth may have increased or decreased since the information
was furnished, and there may be additional Selling Holders of which the Company
is unaware.
In view of the fact that Selling Holders may offer all or a portion of the
Notes or shares of Common Stock held by them pursuant to this offering, and
because this offering is not being underwritten on a firm commitment basis, no
estimate can be given as to the amount of Notes or the number of shares of
Common Stock that will be held by the Selling Holders after completion of this
offering. In addition, the Selling Holders identified in the Prospectus and each
Prospectus Supplement may have sold, transferred or otherwise disposed of all or
a portion of their Notes in transactions exempt from registration under the
Securities Act since the date on which they provided information regarding their
Notes.
