<PAGE>

                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D. C. 20549
                           --------------------------

                                    FORM 8-K

                                 CURRENT REPORT

                       Pursuant to Section 13 or 15(d) of
                       The Securities Exchange Act of 1934
                            -------------------------

         Date of Report (Date of earliest event reported): May 15, 2006

                          MERRILL LYNCH DEPOSITOR, INC.
                     (ON BEHALF OF PPLUS TRUST SERIES SPR-1)
             (Exact name of registrant as specified in its charter)

             DELAWARE             001-31912                    13-3891329
         (State or other        (Commission                (I. R. S. Employer
         jurisdiction  of        File Number)              Identification No.)
          incorporation)

      WORLD FINANCIAL CENTER,                                     10080
        NEW YORK, NEW YORK                                     (Zip Code)
       (Address of principal
        executive offices)
                           --------------------------

       Registrant's telephone number, including area code: (212) 449-1000

                      INFORMATION TO BE INCLUDED IN REPORT


Check the appropriate box below if the Form 8-K filing is intended to
simultaneously satisfy the filing obligation of the registrant under any of the
following provisions (see General Instruction A.2.):

[  ]  Written communications pursuant to Rule 425 under the Securities Act (17
      CFR 230.425)

[  ]  Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR
      240.14a-12)

[  ]  Pre-commencement communications pursuant to Rule 14d-2(b) under the
      Exchange Act (17 CFR 240.14d-2(b))

[  ]  Pre-commencement communications pursuant to Rule 13e-4(c) under the
      Exchange Act (17 CFR 240.13e-4(c))




SECTION 1.   REGISTRANT'S BUSINESS AND OPERATIONS

             Not applicable.

SECTION 2.   FINANCIAL INFORMATION

             Not applicable.

SECTION 3.   SECURITIES AND TRADING MARKETS

             Not applicable.

SECTION 4.   MATTERS RELATED TO ACCOUNTANTS AND FINANCIAL STATEMENTS

             Not applicable.

SECTION 5.   CORPORATE GOVERNANCE AND MANAGEMENT

             Not applicable.

SECTION 6.   ASSET-BACKED SECURITIES

             Not applicable.

SECTION 7.   REGULATION FD

             Not applicable.

SECTION 8.   OTHER EVENTS

         ITEM 8.01     OTHER EVENTS

              99.1     Distribution to holders of the PPLUS Trust
                       Certificates Series SPR-1 on May 15, 2006.

                             For information with respect to the underlying
                       securities held by PPLUS Trust Series SPR-1, please refer
                       to Sprint Nextel Corporation's (Commission file number
                       001-04721) periodic reports, including annual reports on
                       Form 10-K, quarterly reports on Form 10-Q and current
                       reports on Form 8-K, and other information on file with
                       the Securities and Exchange Commission (the "SEC"). You
                       can read and copy these reports and other information at
                       the public reference facilities maintained by the SEC at
                       Room 1580, 100 F Street, NE, Washington, D.C. 20549. You
                       may obtain copies of this material for a fee by writing
                       to the SEC's Public Reference Section of the SEC at 100 F
                       Street, NE, Washington, D.C. 20549. You may obtain
                       information about the operation of the Public Reference
                       Room by calling the SEC at 1-800-SEC-0330. You can also
                       access some of this information electronically by means
                       of the SEC's website on the Internet at
                       http://www.sec.gov, which contains reports, proxy and
                       information statements and other information that the
                       underlying securities issuer has filed electronically
                       with the SEC.


<PAGE>

                             Although we have no reason to believe the
                       information concerning the underlying securities, the
                       underlying guarantee, the underlying securities issuer or
                       the underlying securities guarantor contained in the
                       underlying securities guarantor's Exchange Act reports is
                       not reliable, neither the depositor nor the trustee
                       participated in the preparation of such documents or made
                       any due diligence inquiry with respect to the information
                       provided therein. No investigation with respect to the
                       underlying securities issuer or the underlying securities
                       guarantor (including, without limitation, no
                       investigation as to their financial condition or
                       creditworthiness) or of the underlying securities or the
                       underlying guarantee has been made. You should obtain and
                       evaluate the same information concerning the underlying
                       securities issuer and the underlying securities guarantor
                       as you would obtain and evaluate if your investment were
                       directly in the underlying securities or in other
                       securities issued by the underlying securities issuer or
                       the underlying securities guarantor. There can be no
                       assurance that events affecting the underlying securities
                       and the underlying guarantee or the underlying securities
                       issuer and the underlying securities guarantor have not
                       occurred or have not yet been publicly disclosed which
                       would affect the accuracy or completeness of the publicly
                       available documents described above. .

SECTION 9.   FINANCIAL STATEMENTS AND EXHIBITS

         ITEM 9.01     FINANCIAL STATEMENTS AND EXHIBITS

               (a)     Financial statements of business acquired.

                       Not Applicable.

               (b)     Pro forma financial information.

                       Not Applicable.

               (c)     Shell company transactions.

                       Not Applicable.

               (d)     Exhibits.

                       99.1  Trustee's report in respect of the May 15, 2006
                             distribution to holders of the PPLUS Trust
                             Certificates Series SPR-1.


<PAGE>



                                   SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the
registrant has duly caused this report to be signed on its behalf by the
undersigned, hereunto duly authorized.


                                            MERRILL LYNCH DEPOSITOR, INC.

Date: 06/05/2006                            By:    /s/ Zvi Gillon
                                                    ----------------------------
                                            Name:  Zvi Gillon
                                            Title: Vice President




<PAGE>


                                  EXHIBIT INDEX


99.1     Trustee's report in respect of the May 15, 2006 distribution to holders
         of the PPLUS Trust Certificates Series SPR-1.
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SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D. C. 20549
                           --------------------------

                                    FORM 8-K

                                 CURRENT REPORT

                       Pursuant to Section 13 or 15(d) of
                       The Securities Exchange Act of 1934
                            -------------------------

         Date of Report (Date of earliest event reported): May 15, 2006

                          MERRILL LYNCH DEPOSITOR, INC.
                     (ON BEHALF OF PPLUS TRUST SERIES SPR-1)
             (Exact name of registrant as specified in its charter)

             DELAWARE             001-31912                    13-3891329
         (State or other        (Commission                (I. R. S. Employer
         jurisdiction  of        File Number)              Identification No.)
          incorporation)

      WORLD FINANCIAL CENTER,                                     10080
        NEW YORK, NEW YORK                                     (Zip Code)
       (Address of principal
        executive offices)
                           --------------------------

       Registrant's telephone number, including area code: (212) 449-1000

                      INFORMATION TO BE INCLUDED IN REPORT

Check the appropriate box below if the Form 8-K filing is intended to
simultaneously satisfy the filing obligation of the registrant under any of the
following provisions (see General Instruction A.2.):

[  ]  Written communications pursuant to Rule 425 under the Securities Act (17
      CFR 230.425)

[  ]  Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR
      240.14a-12)

[  ]  Pre-commencement communications pursuant to Rule 14d-2(b) under the
      Exchange Act (17 CFR 240.14d-2(b))

[  ]  Pre-commencement communications pursuant to Rule 13e-4(c) under the
      Exchange Act (17 CFR 240.13e-4(c))

SECTION 1.   REGISTRANT'S BUSINESS AND OPERATIONS

             Not applicable.

SECTION 2.   FINANCIAL INFORMATION

             Not applicable.

SECTION 3.   SECURITIES AND TRADING MARKETS

             Not applicable.

SECTION 4.   MATTERS RELATED TO ACCOUNTANTS AND FINANCIAL STATEMENTS

             Not applicable.

SECTION 5.   CORPORATE GOVERNANCE AND MANAGEMENT

             Not applicable.

SECTION 6.   ASSET-BACKED SECURITIES

             Not applicable.

SECTION 7.   REGULATION FD

             Not applicable.

SECTION 8.   OTHER EVENTS

         ITEM 8.01     OTHER EVENTS

              99.1     Distribution to holders of the PPLUS Trust
                       Certificates Series SPR-1 on May 15, 2006.

                             For information with respect to the underlying
                       securities held by PPLUS Trust Series SPR-1, please refer
                       to Sprint Nextel Corporation's (Commission file number
                       001-04721) periodic reports, including annual reports on
                       Form 10-K, quarterly reports on Form 10-Q and current
                       reports on Form 8-K, and other information on file with
                       the Securities and Exchange Commission (the "SEC"). You
                       can read and copy these reports and other information at
                       the public reference facilities maintained by the SEC at
                       Room 1580, 100 F Street, NE, Washington, D.C. 20549. You
                       may obtain copies of this material for a fee by writing
                       to the SEC's Public Reference Section of the SEC at 100 F
                       Street, NE, Washington, D.C. 20549. You may obtain
                       information about the operation of the Public Reference
                       Room by calling the SEC at 1-800-SEC-0330. You can also
                       access some of this information electronically by means
                       of the SEC's website on the Internet at
                       http://www.sec.gov, which contains reports, proxy and
                       information statements and other information that the
                       underlying securities issuer has filed electronically
                       with the SEC.
Although we have no reason to believe the
                       information concerning the underlying securities, the
                       underlying guarantee, the underlying securities issuer or
                       the underlying securities guarantor contained in the
                       underlying securities guarantor's Exchange Act reports is
                       not reliable, neither the depositor nor the trustee
                       participated in the preparation of such documents or made
                       any due diligence inquiry with respect to the information
                       provided therein. No investigation with respect to the
                       underlying securities issuer or the underlying securities
                       guarantor (including, without limitation, no
                       investigation as to their financial condition or
                       creditworthiness) or of the underlying securities or the
                       underlying guarantee has been made. You should obtain and
                       evaluate the same information concerning the underlying
                       securities issuer and the underlying securities guarantor
                       as you would obtain and evaluate if your investment were
                       directly in the underlying securities or in other
                       securities issued by the underlying securities issuer or
                       the underlying securities guarantor. There can be no
                       assurance that events affecting the underlying securities
                       and the underlying guarantee or the underlying securities
                       issuer and the underlying securities guarantor have not
                       occurred or have not yet been publicly disclosed which
                       would affect the accuracy or completeness of the publicly
                       available documents described above. .

SECTION 9.   FINANCIAL STATEMENTS AND EXHIBITS

         ITEM 9.01     FINANCIAL STATEMENTS AND EXHIBITS

               (a)     Financial statements of business acquired.

                       Not Applicable.

               (b)     Pro forma financial information.

                       Not Applicable.

               (c)     Shell company transactions.

                       Not Applicable.

               (d)     Exhibits.

                       99.1  Trustee's report in respect of the May 15, 2006
                             distribution to holders of the PPLUS Trust
                             Certificates Series SPR-1.
SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the
registrant has duly caused this report to be signed on its behalf by the
undersigned, hereunto duly authorized.

                                            MERRILL LYNCH DEPOSITOR, INC.

Date: 06/05/2006                            By:    /s/ Zvi Gillon
                                                    ----------------------------
                                            Name:  Zvi Gillon
                                            Title: Vice President
EXHIBIT INDEX

99.1     Trustee's report in respect of the May 15, 2006 distribution to holders
         of the PPLUS Trust Certificates Series SPR-1.