diff --git a/google/spanner/v1/transaction.proto b/google/spanner/v1/transaction.proto
index 3577f797cbf41..49ab5cf5235aa 100644
--- a/google/spanner/v1/transaction.proto
+++ b/google/spanner/v1/transaction.proto
@@ -56,7 +56,8 @@ option ruby_package = "Google::Cloud::Spanner::V1";
 //
 //      Queries on change streams must be performed with the snapshot read-only
 //      transaction mode, specifying a strong read. Please see
-//      [TransactionOptions.ReadOnly.strong][google.spanner.v1.TransactionOptions.ReadOnly.strong] for more details.
+//      [TransactionOptions.ReadOnly.strong][google.spanner.v1.TransactionOptions.ReadOnly.strong]
+//      for more details.
 //
 //   3. Partitioned DML. This type of transaction is used to execute
 //      a single Partitioned DML statement. Partitioned DML partitions
@@ -192,7 +193,8 @@ option ruby_package = "Google::Cloud::Spanner::V1";
 // Queries on change streams (see below for more details) must also specify
 // the strong read timestamp bound.
 //
-// See [TransactionOptions.ReadOnly.strong][google.spanner.v1.TransactionOptions.ReadOnly.strong].
+// See
+// [TransactionOptions.ReadOnly.strong][google.spanner.v1.TransactionOptions.ReadOnly.strong].
 //
 // Exact staleness:
 //
@@ -213,7 +215,9 @@ option ruby_package = "Google::Cloud::Spanner::V1";
 // equivalent boundedly stale concurrency modes. On the other hand,
 // boundedly stale reads usually return fresher results.
 //
-// See [TransactionOptions.ReadOnly.read_timestamp][google.spanner.v1.TransactionOptions.ReadOnly.read_timestamp] and
+// See
+// [TransactionOptions.ReadOnly.read_timestamp][google.spanner.v1.TransactionOptions.ReadOnly.read_timestamp]
+// and
 // [TransactionOptions.ReadOnly.exact_staleness][google.spanner.v1.TransactionOptions.ReadOnly.exact_staleness].
 //
 // Bounded staleness:
@@ -243,7 +247,9 @@ option ruby_package = "Google::Cloud::Spanner::V1";
 // which rows will be read, it can only be used with single-use
 // read-only transactions.
 //
-// See [TransactionOptions.ReadOnly.max_staleness][google.spanner.v1.TransactionOptions.ReadOnly.max_staleness] and
+// See
+// [TransactionOptions.ReadOnly.max_staleness][google.spanner.v1.TransactionOptions.ReadOnly.max_staleness]
+// and
 // [TransactionOptions.ReadOnly.min_read_timestamp][google.spanner.v1.TransactionOptions.ReadOnly.min_read_timestamp].
 //
 // Old read timestamps and garbage collection:
@@ -349,14 +355,34 @@ message TransactionOptions {
   // Message type to initiate a read-write transaction. Currently this
   // transaction type has no options.
   message ReadWrite {
+    // `ReadLockMode` is used to set the read lock mode for read-write
+    // transactions.
+    enum ReadLockMode {
+      // Default value.
+      //
+      // If the value is not specified, the pessimistic read lock is used.
+      READ_LOCK_MODE_UNSPECIFIED = 0;
 
-  }
+      // Pessimistic lock mode.
+      //
+      // Read locks are acquired immediately on read.
+      PESSIMISTIC = 1;
 
-  // Message type to initiate a Partitioned DML transaction.
-  message PartitionedDml {
+      // Optimistic lock mode.
+      //
+      // Locks for reads within the transaction are not acquired on read.
+      // Instead the locks are acquired on a commit to validate that
+      // read/queried data has not changed since the transaction started.
+      OPTIMISTIC = 2;
+    }
 
+    // Read lock mode for the transaction.
+    ReadLockMode read_lock_mode = 1;
   }
 
+  // Message type to initiate a Partitioned DML transaction.
+  message PartitionedDml {}
+
   // Message type to initiate a read-only transaction.
   message ReadOnly {
     // How to choose the timestamp for the read-only transaction.
@@ -421,7 +447,8 @@ message TransactionOptions {
     }
 
     // If true, the Cloud Spanner-selected read timestamp is included in
-    // the [Transaction][google.spanner.v1.Transaction] message that describes the transaction.
+    // the [Transaction][google.spanner.v1.Transaction] message that describes
+    // the transaction.
     bool return_read_timestamp = 6;
   }
 
@@ -475,7 +502,8 @@ message Transaction {
 // [Read][google.spanner.v1.Spanner.Read] or
 // [ExecuteSql][google.spanner.v1.Spanner.ExecuteSql] call runs.
 //
-// See [TransactionOptions][google.spanner.v1.TransactionOptions] for more information about transactions.
+// See [TransactionOptions][google.spanner.v1.TransactionOptions] for more
+// information about transactions.
 message TransactionSelector {
   // If no fields are set, the default is a single use transaction
   // with strong concurrency.
@@ -490,7 +518,8 @@ message TransactionSelector {
 
     // Begin a new transaction and execute this read or SQL query in
     // it. The transaction ID of the new transaction is returned in
-    // [ResultSetMetadata.transaction][google.spanner.v1.ResultSetMetadata.transaction], which is a [Transaction][google.spanner.v1.Transaction].
+    // [ResultSetMetadata.transaction][google.spanner.v1.ResultSetMetadata.transaction],
+    // which is a [Transaction][google.spanner.v1.Transaction].
     TransactionOptions begin = 3;
   }
 }