Arcot Attribute "USERNAME" is by default mapped to LDAP repository attribute "uid" and can neither be un-mapped for the LDAP Organization via the Admin UI nor can it be updated via the updateOrg Web Service call.
1. Here is a screen shot of the error received when trying to un-map "USERNAME=uid" for a LDAP organization (via the Admin UI). The error indicates "USERNAME" cannot be un-mapped".
<Please see attached file for image>

2. Here is the Web Service response once gets when one tries to update the Arcot Attribute to LDAP attribute "USERNAME=uid".
Error 31157 - Invalid arcot attribute, USERNAME"<?xml version='1.0' encoding='UTF-8'?><soapenv:Envelope xmlns:soapenv="http://www.w3.org/2003/05/soap-envelope"><soapenv:Header><udsTransactionID xmlns="http://ws.arcot.com/UDSTransaction/1.0">334bc9a7-f980-4d9b-8803-d0d955912db2</udsTransactionID><authToken xmlns="http://ws.arcot.com/UDSTransaction/1.0"/></soapenv:Header><soapenv:Body><soapenv:Fault><soapenv:Code><soapenv:Value>soapenv:Receiver</soapenv:Value></soapenv:Code><soapenv:Reason><soapenv:Text xml:lang="en-US">Invalid arcot attribute, USERNAME </soapenv:Text></soapenv:Reason><soapenv:Detail><ns2:orgRegistryError xmlns:ns2="http://ws.arcot.com/ArcotUserRegistryMgmtSvc/1.0/msgs">
<ns1:errorMessage xmlns:ns1="http://ws.arcot.com/UserSchema/1.0">Invalid arcot attribute, USERNAME </ns1:errorMessage><ns1:errorNumber xmlns:ns1="http://ws.arcot.com/UserSchema/1.0">31157</ns1:errorNumber></ns2:orgRegistryError></soapenv:Detail></soapenv:Fault></soapenv:Body></soapenv:Envelope>